Varenicline attenuates nicotine-enhanced brain-stimulation reward by activation of alpha4beta2 nicotinic receptors in rats

Cigarette Smoke Extract, but Not Electronic Cigarette Aerosol Extract, Inhibits Monoamine Oxidase in vitro and Produces Greater Acute Aversive/Anhedonic Effects Than Nicotine Alone on Intracranial Self-Stimulation in Rats

Conventional tobacco cigarettes appear to have greater abuse liability than non-combusted products such as electronic cigarettes (ECs) and nicotine replacement therapy (NRT). This may be due to the higher levels of behaviorally active non-nicotine constituents [e.g., monoamine oxidase (MAO) inhibitors such as β-carbolines] in cigarette smoke (CS) compared to non-combusted products. To evaluate this hypothesis, the current studies compared the relative abuse liability of CS and EC aerosol extracts containing nicotine and a range of non-nicotine constituents to that of nicotine alone (NRT analog) using intracranial self-stimulation (ICSS) in rats. Effects of formulations on brain MAO activity in vitro and ex vivo were also studied to evaluate the potential role of MAO inhibition in the ICSS study. CS extract contained higher levels of several behaviorally active non-nicotine constituents (e.g., the β-carbolines norharmane and harmane) than EC extract. Nicotine alone reduced ICSS thresholds at a moderate nicotine dose, suggesting a reinforcement-enhancing effect that may promote abuse liability, and elevated ICSS thresholds at a high nicotine dose, suggesting an aversive/anhedonic effect that may limit abuse liability. CS extract elevated ICSS thresholds to a greater degree than nicotine alone at high nicotine doses. Effects of EC extract on ICSS did not differ from those of nicotine alone. Finally, CS extract significantly inhibited MAO-A and MAO-B activity in vitro, whereas EC extract and nicotine alone did not. None of the formulations inhibited MAO measured ex vivo. These findings indicate greater acute aversive/anhedonic effects for CS extract compared to nicotine alone, suggesting lower abuse liability. Although confirmation of our findings using other dosing regimens, preclinical addiction models, and tobacco product extracts is needed, these findings suggest that the centrally-mediated effects of MAO inhibitors and other non-nicotine constituents may not account for the greater abuse liability of cigarettes compared to non-combusted products. Nonetheless, identifying the specific constituent(s) mediating the effects of CS extracts in this study could help clarify mechanisms mediating tobacco addiction and inform FDA product standards.

Nicotine-induced enhancement of a sensory reinforcer in adult rats: antagonist pretreatment effects

RATIONALE AND OBJECTIVES

The reinforcement-enhancing effect (REE) of nicotine refers to the drug's ability to enhance the strength of other primary and conditioned reinforcers. The main aim was to investigate neuropharmacological mechanisms underlying nicotine's strengthening of a primary visual reinforcer (i.e., a light cue), using a subcutaneous (SC) dose previously shown to provide plasma nicotine levels associated with habitual smoking.

METHODS

Adult male rats pressed an "active" lever to illuminate a brief cue light during daily 60-min sessions. Rats that showed a clear REE were tested with systemically administered pretreatment drugs followed by nicotine (0.1 mg/kg SC) or saline challenge, in within-subject counterbalanced designs. Pretreatments were mecamylamine (nicotinic, 0.1-1 mg/kg SC), SCH 39166 (D1-like dopaminergic, 0.003-0.2 mg/kg SC), naloxone (opioid, 1 and 5 mg/kg SC), prazosin (alpha1-adrenergic antagonist, 1 and 2 mg/kg IP), rimonabant (CB1 cannabinoid inverse agonist, 3 mg/kg IP), sulpiride (D2-like dopaminergic antagonist, 40 mg/kg SC), or propranolol (beta-adrenergic antagonist, 10 mg/kg IP).

RESULTS

The nicotine REE was abolished by three antagonists at doses that did not impact motor output, i.e., mecamylamine (1 mg/kg), SCH 39166 (0.01 and 0.03 mg/kg), and naloxone (5 mg/kg). Prazosin and rimonabant both attenuated the nicotine REE, but rimonabant also suppressed responding more generally. The nicotine REE was not significantly altered by sulpiride or propranolol.

CONCLUSIONS

In adult male rats, the reinforcement-enhancing effect of low-dose nicotine depends on nicotinic receptor stimulation and on neurotransmission via D1/D5 dopaminergic, opioid, alpha1-adrenergic, and CB1 cannabinoid receptors.

β-Carbolines found in cigarette smoke elevate intracranial self-stimulation thresholds in rats

Identifying novel constituents that contribute to tobacco addiction is essential for developing more effective treatments and informing FDA regulation of tobacco products. While preclinical data indicate that monoamine oxidase (MAO) inhibitors can have abuse liability or potentiate the addiction-related effects of nicotine, most of these studies have used clinical MAO inhibitors (e.g., tranylcypromine) that are not present in cigarette smoke. The primary goal of this study was to evaluate the abuse potential of the β-carbolines harmane, norharmane, and harmine - MAO inhibitors that are found in cigarette smoke - in an intracranial self-simulation (ICSS) model in rats. A secondary goal was to evaluate the ability of norharmane to influence nicotine's acute effects on ICSS. None of the β-carbolines lowered ICSS thresholds at any dose studied when administered alone, suggesting a lack of abuse liability. Rather, all three β-carbolines produced dose-dependent elevations in ICSS thresholds, indicating aversive/anhedonic effects. Harmane and harmine also elevated ICSS response latencies, suggesting a disruption of motor function, albeit with reduced potency compared to their ICSS threshold-elevating effects. Norharmane (2.5 mg/kg) modestly attenuated the effects of nicotine on ICSS thresholds. Our findings indicate that these β-carbolines produced only aversive/anhedonic effects in an ICSS model when administered alone, and that norharmane unexpectedly attenuated nicotines acute effects on ICSS. Future work evaluating the addiction-related effects of nicotine combined with these and other MAO inhibitors present in smoke may be useful for understanding the role of MAO inhibition in tobacco addiction and informing FDA tobacco regulation.

More than Smoke and Patches: The Quest for Pharmacotherapies to Treat Tobacco Use Disorder

Tobacco use is a persistent public health issue. It kills up to half its users and is the cause of nearly 90% of all lung cancers. The main psychoactive component of tobacco is nicotine, primarily responsible for its abuse-related effects. Accordingly, most pharmacotherapies for smoking cessation target nicotinic acetylcholine receptors (nAChRs), nicotine's major site of action in the brain. The goal of the current review is twofold: first, to provide a brief overview of the most commonly used behavioral procedures for evaluating smoking cessation pharmacotherapies and an introduction to pharmacokinetic and pharmacodynamic properties of nicotine important for consideration in the development of new pharmacotherapies; and second, to discuss current and potential future pharmacological interventions aimed at decreasing tobacco use. Attention will focus on the potential for allosteric modulators of nAChRs to offer an improvement over currently approved pharmacotherapies. Additionally, given increasing public concern for the potential health consequences of using electronic nicotine delivery systems, which allow users to inhale aerosolized solutions as an alternative to smoking tobacco, an effort will be made throughout this review to address the implications of this relatively new form of nicotine delivery, specifically as it relates to smoking cessation. SIGNIFICANCE STATEMENT: Despite decades of research that have vastly improved our understanding of nicotine and its effects on the body, only a handful of pharmacotherapies have been successfully developed for use in smoking cessation. Thus, investigation of alternative pharmacological strategies for treating tobacco use disorder remains active; allosteric modulators of nicotinic acetylcholine receptors represent one class of compounds currently under development for this purpose.

The effects of varenicline on lumbar spinal fusion in a rat model

BACKGROUND CONTEXT

Smoking is detrimental to obtaining a solid spinal fusion mass with previous studies demonstrating its association with pseudoarthrosis in patients undergoing spinal fusion. Varenicline is a pharmacologic adjunct used in smoking cessation which acts as a partial agonist of the same nicotinic receptors activated during tobacco use. However, no clinical or basic science studies to date have characterized if varenicline has negative effects on spinal fusion and bone healing by itself.

PURPOSE

Our study's aim was to elucidate whether varenicline affects the frequency or quality of posterolateral spinal fusion in a rodent model at an endpoint of 12 weeks.

STUDY DESIGN

Randomized control trial.

PATIENT SAMPLE

Fourteen male Lewis rats randomly separated into two experimental groups.

OUTCOME MEASURES

Manual palpation of fusion segment, radiography, μCT imaging, and four-point bend.

METHODS

Fourteen male Lewis rats were randomly separated into two experimental groups undergoing L4-L5 posterior spinal fusion procedure followed by daily subcutaneous injections of human dose varenicline or saline (control) for 12 weeks postsurgery. Spine samples were explanted, and fusion was determined via manual palpation of segments by two independent observers. High-resolution radiographs were obtained to evaluate bridging fusion mass. μCT imaging was performed to characterize fusion mass and consolidation. Lumbar spinal fusion units were tested in four-point bending to evaluate stiffness and peak load. Study funding sources include $5000 OREF Grant. There were no applicable financial relationships or conflicts of interest.

RESULTS

At 3 months postsurgery, 12 out of 14 rats demonstrated lumbar spine fusion (86% fused) with no difference in fusion frequency between the varenicline and control groups as detected by manual palpation. High-resolution radiography revealed six out of seven rats (86%) having complete fusion in both groups. μCT showed no significant difference in bone mineral density or bone fraction volume between groups in the region of interest. Biomechanical testing demonstrated no significant different in the average stiffness or peak loads at the fusion site of the varenicline and control groups.

CONCLUSION

Based on the results of our rat study, there is no indication that varenicline itself has a detrimental effect on the frequency and quality of spinal fusion.

Cannabinoid CB1 and CB2 receptor mechanisms underlie cannabis reward and aversion in rats

BACKGROUND AND PURPOSE

Endocannabinoids are critically involved in brain reward functions, mediated by activation of CB₁ receptors, reflecting their high density in the brain. However, the recent discovery of CB₂ receptors in the brain, particularly in the midbrain dopamine neurons, has challenged this view and inspired us to re-examine the roles of both CB₁ and CB₂ receptors in the effects of cannabis.

EXPERIMENTAL APPROACH

In the present study, we used the electrical intracranial self-stimulation paradigm to evaluate the effects of various cannabinoid drugs on brain reward in laboratory rats and the roles of CB₁ and CB₂ receptors activation in brain reward function(s).

KEY RESULTS

Two mixed CB₁ / CB₂ receptor agonists, Δ⁹ -tetrahydrocannabinol (Δ⁹ -THC) and WIN55,212-2, produced biphasic effects-mild enhancement of brain-stimulation reward (BSR) at low doses but inhibition at higher doses. Pretreatment with a CB₁ receptor antagonist (AM251) attenuated the low dose-enhanced BSR, while a CB₂ receptor antagonist (AM630) attenuated high dose-inhibited BSR. To confirm these opposing effects, rats were treated with selective CB₁ and CB₂ receptor agonists. These compounds produced significant BSR enhancement and inhibition, respectively.

CONCLUSIONS AND IMPLICATIONS

CB₁ receptor activation produced reinforcing effects, whereas CB₂ receptor activation was aversive. The subjective effects of cannabis depend on the balance of these opposing effects. These findings not only explain previous conflicting results in animal models of addiction but also explain why cannabis can be either rewarding or aversive in humans, as expression of CB₁ and CB₂ receptors may differ in the brains of different subjects.

Status and Future Directions of Preclinical Behavioral Pharmacology in Tobacco Regulatory Science

Behavioral pharmacology is a branch of the experimental analysis of behavior that has had great influence in drug addiction research and policy. This paper provides an overview of recent behavioral pharmacology research in the field of tobacco regulatory science, which provides the scientific foundation for the Food and Drug Administration Center for Tobacco Products (FDA CTP) to set tobacco control policies. The rationale and aims of tobacco regulatory science are provided, including the types of preclinical operant behavioral models it deems important for assessing the abuse liability of tobacco products and their constituents. We then review literature relevant to key regulatory actions being considered by the FDA CTP, including regulations over nicotine and menthol content of cigarettes, and conclude with suggesting some directions for future research. The current era of tobacco regulatory science provides great opportunities for behavioral pharmacologists to address the leading cause of preventable death and disease worldwide.

Discovery and development of varenicline for smoking cessation

INTRODUCTION

Tobacco use causes one premature death every six seconds. Current smoking cessation aids include nicotine replacement therapies, bupropion, and varenicline. Although more than 70% of smokers express a desire to quit, fewer than 3% remain abstinent for more than one year, highlighting a critical need for more efficacious smoking cessation treatments. Areas covered: The authors discuss the rationale, preclinical and clinical development of varenicline for smoking cessation. They cover the development of varenicline as a partial agonist at α4β2 receptors, the primary neural substrate for nicotine reward. Then, they discuss evidence from preclinical studies indicating varenicline's efficacy in blocking nicotine reward, followed by clinical trials demonstrating safety and efficacy in sustaining abstinence in smokers. Finally, they cover post-market surveillance, including caution in heavy machine operators, putative cardiovascular risk, and the repealed warning for adverse neuropsychiatric events. Expert opinion: Varenicline development was based on strong theoretical rationale and preclinical evidence. Clinical studies indicate that varenicline is safe and more effective in sustaining abstinence than placebo, bupropion or nicotine replacement therapies. However, given that continuous abstinence rates across studies remain low (18 ~ 30% with varenicline; 4 ~ 10% with placebo), novel and more effective medications targeting other nicotinic or glutamate receptors for smoking cessation are required.

Effects of nicotine-containing and "nicotine-free" e-cigarette refill liquids on intracranial self-stimulation in rats

BACKGROUND

Animal models are needed to inform FDA regulation of electronic cigarettes (ECs) because they avoid limitations associated with human studies. We previously reported that an EC refill liquid produced less aversive/anhedonic effects at a high nicotine dose than nicotine alone as measured by elevations in intracranial self-stimulation (ICSS) thresholds, which may reflect the presence of behaviorally active non-nicotine constituents (e.g., propylene glycol) in the EC liquids. The primary objective of this study was to assess the generality of our prior ICSS findings to two additional EC liquids. We also compared effects of "nicotine-free" varieties of these EC liquids on ICSS, as well as binding affinity and/or functional activity of nicotine alone, nicotine-containing EC liquids, and "nicotine-free" EC liquids at nicotinic acetylcholine receptors (nAChRs).

METHODS AND RESULTS

Nicotine alone and nicotine dose-equivalent concentrations of both nicotine-containing EC liquids produced similar lowering of ICSS thresholds at low to moderate nicotine doses, indicating similar reinforcement-enhancing effects. At high nicotine doses, nicotine alone elevated ICSS thresholds (a measure of anhedonia-like behavior) while the EC liquids did not. Nicotine-containing EC liquids did not differ from nicotine alone in terms of binding affinity or functional activity at nAChRs. "Nicotine-free" EC liquids did not affect ICSS, but bound with low affinity at some (e.g., α4ß2) nAChRs.

CONCLUSIONS

These findings suggest that non-nicotine constituents in these EC liquids do not contribute to their reinforcement-enhancing effects. However, they may attenuate nicotine's acute aversive/anhedonic and/or toxic effects, which may moderate the abuse liability and/or toxicity of ECs.

Tobacco's minor alkaloids: Effects on place conditioning and nucleus accumbens dopamine release in adult and adolescent rats

Tobacco products are some of the most commonly used psychoactive drugs worldwide. Besides nicotine, alkaloids in tobacco include cotinine, myosmine, and anatabine. Scientific investigation of these constituents and their contribution to tobacco dependence is less well developed than for nicotine. The present study evaluated the nucleus accumbens dopamine-releasing properties and rewarding and/or aversive properties of nicotine (0.2-0.8mg/kg), cotinine (0.5-5.0mg/kg), anatabine (0.5-5.0mg/kg), and myosmine (5.0-20.0mg/kg) through in vivo microdialysis and place conditioning, respectively, in adult and adolescent male rats. Nicotine increased dopamine release at both ages, and anatabine and myosmine increased dopamine release in adults, but not adolescents. The dopamine release results were not related to place conditioning, as nicotine and cotinine had no effect on place conditioning, whereas anatabine and myosmine produced aversion in both ages. While the nucleus accumbens shell is hypothesized to play a role in strengthening drug-context associations following initiation of drug use, it may have little involvement in the motivational effects of tobacco constituents once these associations have been acquired. Effects of myosmine and anatabine on dopamine release may require a fully developed dopamine system, since no effects of these tobacco alkaloids were observed during adolescence. In summary, while anatabine and myosmine-induced dopamine release in nucleus accumbens may play a role in tobacco dependence in adults, the nature of that role remains to be elucidated.

Electrophysiology-Based Assays to Detect Subtype-Selective Modulation of Human Nicotinic Acetylcholine Receptors

The Family Smoking Prevention and Tobacco Control Act of 2009 (Public Law 111-31) gave the US Food and Drug Administration (FDA) the responsibility for regulating tobacco products. Nicotine is the primary addictive component of tobacco and its effects can be modulated by additional ingredients in manufactured products. Nicotine acts by mimicking the neurotransmitter acetylcholine on neuronal nicotinic acetylcholine receptors (nAChRs), which function as ion channels in cholinergic modulation of neurotransmission. Subtypes within the family of neuronal nAChRs are defined by their α- and β-subunit composition. The subtype-selective profiles of tobacco constituents are largely unknown, but could be essential for understanding the physiological effects of tobacco products. In this report, we report the development and validation of electrophysiology-based high-throughput screens (e-HTS)for human nicotinic subtypes, α3β4, α3β4α5, α4β2, and α7 stably expressed in Chinese Hamster Ovary cells. Assessment of agonist sensitivity and acute desensitization gave results comparable to those obtained by conventional manual patch clamp electrophysiology assays. The potency of reference antagonists for inhibition of the receptor channels and selectivity of positive allosteric modulators also were very similar between e-HTS and conventional manual patch voltage clamp data. Further validation was obtained in pilot screening of a library of FDA-approved drugs that identified α7 subtype-selective positive allosteric modulation by novel compounds. These assays provide new tools for profiling of nicotinic receptor selectivity.

Similar precipitated withdrawal effects on intracranial self-stimulation during chronic infusion of an e-cigarette liquid or nicotine alone

The FDA recently extended their regulatory authority to electronic cigarettes (ECs). Because the abuse liability of ECs is a leading concern of the FDA, animal models are urgently needed to identify factors that influence the relative abuse liability of these products. The ability of tobacco products to induce nicotine dependence, defined by the emergence of anhedonia and other symptoms of nicotine withdrawal following cessation of their use, contributes to tobacco abuse liability. The present study compared the severity of precipitated withdrawal during chronic infusion of nicotine alone or nicotine-dose equivalent concentrations of three different EC refill liquids in rats, as indicated by elevations in intracranial self-stimulation (ICSS) thresholds (anhedonia-like behavior). Because these EC liquids contain constituents that may enhance their abuse liability (e.g., minor alkaloids), we hypothesized that they would be associated with greater withdrawal effects than nicotine alone. Results indicated that the nicotinic acetylcholine receptor antagonist mecamylamine precipitated elevations in ICSS thresholds in rats receiving a chronic infusion of nicotine alone or EC liquids (3.2mg/kg/day, via osmotic pump). Magnitude of this effect did not differ between formulations. Our findings indicate that nicotine alone is the primary CNS determinant of the ability of ECs to engender dependence. Combined with our previous findings that nicotine alone and these EC liquids do not differ in other preclinical addiction models, these data suggest that product standards set by the FDA to reduce EC abuse liability should primarily target nicotine, other constituents with peripheral sensory effects (e.g. flavorants), and factors that influence product appeal (e.g., marketing).

Delayed Varenicline Administration Reduces Inflammation and Improves Forelimb Use Following Experimental Stroke

BACKGROUND

Pharmacological activation of the cholinergic anti-inflammatory pathway (CAP), specifically by activating α7 nicotinic acetylcholine receptors, has been shown to confer short-term improvements in outcome. Most studies have investigated administration within 24 hours of stroke, and few have investigated drugs approved for use in human patients. We investigated whether delayed administration of varenicline, a high-affinity agonist at α7 nicotinic receptors and an established therapy for nicotine addiction, decreased brain inflammation and improved functional performance in a mouse model of experimental stroke.

METHODS

CSF-1R-EGFP (MacGreen) mice were subjected to transient middle cerebral artery occlusion and administered varenicline (2.5 mg/kg/d for 7 days) or saline (n = 10 per group) 3 days after stroke. Forelimb asymmetry was assessed in the Cylinder test every 2 days after surgery, and structural lesions were quantified at day 10. Enhanced green fluorescent protein (EGFP) and growth associated protein 43 (GAP43) immunohistochemistry were used to evaluate the effect of varenicline on inflammation and axonal regeneration, respectively.

RESULTS

Varenicline-treated animals showed a significant increase in impaired forelimb use compared with saline-treated animals 10 days after stroke. Varenicline treatment was associated with reduced EGFP expression and increased GAP43 expression in the striatum of MacGreen mice.

CONCLUSION

Our results show that delayed administration of varenicline promotes recovery of function following experimental stroke. Motor function improvements were accompanied by decreased brain inflammation and increased axonal regeneration in nonpenumbral areas. These results suggest that the administration of an exogenous nicotinic agonist in the subacute phase following stroke may be a viable therapeutic strategy for stroke patients.

Preclinical abuse liability assessment of ABT-126, an agonist at the α7 nicotinic acetylcholine receptor (nAChR)

ABT-126 is a nicotinic acetylcholine receptor (nAChR) agonist that is selective for the α7 subtype of the receptor. nAChRs are thought to play a role in a variety of neurocognitive processes and have been a pharmacologic target for disorders with cognitive impairment, including schizophrenia and Alzheimer's disease. As part of the preclinical safety package for ABT-126, its potential for abuse was assessed. While the involvement of the α4β2 subtype of the nicotinic receptor in the addictive properties of nicotine has been demonstrated, the role of the α7 receptor has been studied much less extensively. A number of preclinical assays of abuse potential including open-field, drug discrimination and self-administration were employed in male rats. ABT-126 had modest effects on locomotor activity in the open-field assay. In nicotine and d-amphetamine drug discrimination assays, ABT-126 administration failed to produce appreciable d-amphetamine-like or nicotine-like responding, suggesting that its interoceptive effects are distinct from those of these drugs of abuse. In rats trained to self-administer cocaine, substitution with ABT-126 was similar to substitution with saline, indicating that it lacks reinforcing effects. No evidence of physical dependence was noted following subchronic administration. Overall, these data suggest that ABT-126 has a low potential for abuse. Together with other literature on this drug class, it appears that drugs that selectively activate α7 nAChRs are not likely to result in abuse or dependence.

Abuse liability assessment of an e-cigarette refill liquid using intracranial self-stimulation and self-administration models in rats

BACKGROUND

The popularity of electronic cigarettes (ECs) has increased dramatically despite their unknown health consequences. Because the abuse liability of ECs is one of the leading concerns of the Food and Drug Administration (FDA), models to assess it are urgently needed to inform FDA regulatory decisions regarding these products. The purpose of this study was to assess the relative abuse liability of an EC liquid compared to nicotine alone in rats. Because this EC liquid contains non-nicotine constituents that may enhance its abuse liability, we hypothesized that it would have greater abuse liability than nicotine alone.

METHODS

Nicotine alone and nicotine dose-equivalent concentrations of EC liquid were compared in terms of their acute effects on intracranial self-stimulation (ICSS) thresholds, acquisition of self-administration, reinforcing efficacy (i.e., elasticity of demand), blockade of these behavioral effects by mecamylamine, nicotine pharmacokinetics and nicotinic acetylcholine receptor binding and activation.

RESULTS

There were no significant differences between formulations on any measure, except that EC liquid produced less of an elevation in ICSS thresholds at high nicotine doses.

CONCLUSIONS

Collectively, these findings suggest that the relative abuse liability of this EC liquid is similar to that of nicotine alone in terms of its reinforcing and reinforcement-enhancing effects, but that it may have less aversive/anhedonic effects at high doses. The present methods may be useful for assessing the abuse liability of other ECs to inform potential FDA regulation of those products.

Comparison of effects produced by nicotine and the α4β2-selective agonist 5-I-A-85380 on intracranial self-stimulation in rats

Intracranial self-stimulation (ICSS) is one type of preclinical procedure for research on pharmacological mechanisms that mediate abuse potential of drugs acting at various targets, including nicotinic acetylcholine receptors (nAChRs). This study compared effects of the nonselective nAChR agonist nicotine (0.032-1.0 mg/kg) and the α4β2-selective nAChR agonist 5-I-A-85380 (0.01-1.0 mg/kg) on ICSS in male Sprague-Dawley rats. Rats were implanted with electrodes targeting the medial forebrain bundle at the level of the lateral hypothalamus and trained to respond under a fixed-ratio 1 schedule for a range of brain stimulation frequencies (158-56 Hz). A broad range of 5-I-A-85380 doses produced an abuse-related increase (or "facilitation") of low ICSS rates maintained by low brain-stimulation frequencies, and this effect was blocked by both the nonselective nAChR antagonist mecamylamine and the selective α4β2 antagonist dihyrdo-β-erythroidine (DHβE). Conversely, nicotine produced weaker ICSS facilitation across a narrower range of doses, and higher nicotine doses decreased high rates of ICSS maintained by high brain-stimulation frequencies. The rate-decreasing effects of a high nicotine dose were blocked by mecamylamine but not DHβE. Chronic nicotine treatment produced selective tolerance to rate-decreasing effects of nicotine but did not alter ICSS rate-increasing effects of nicotine. These results suggest that α4β2 receptors are sufficient to mediate abuse-related rate-increasing effects of nAChR agonists in this ICSS procedure. Conversely, nicotine effects at non-α4β2 nAChRs appear to oppose and limit abuse-related effects mediated by α4β2 receptors, although tolerance can develop to these non-α4β2 effects. Selective α4β2 agonists may have higher abuse potential than nicotine.

Effects of nicotinic acetylcholine receptor agonists in assays of acute pain-stimulated and pain-depressed behaviors in rats

Agonists at nicotinic acetylcholine receptors (nAChRs) constitute one drug class being evaluated as candidate analgesics. Previous preclinical studies have implicated α4β2 and α7 nAChRs as potential mediators of the antinociceptive effects of (–)-nicotine hydrogen tartrate (nicotine) and other nAChR agonists; however, these studies have relied exclusively on measures of pain-stimulated behavior, which can be defined as behaviors that increase in frequency, rate, or intensity after presentation of a noxious stimulus. Pain is also associated with depression of many behaviors, and drug effects can differ in assays of pain-stimulated versus pain-depressed behavior. Accordingly, this study compared the effects of nicotine, the selective α4/6β2 agonist 5-(123I)iodo-3-[2(S)-2-azetidinylmethoxy]pyridine (5-I-A-85380), and the selective α7 agonist N-(3R)-1-azabicyclo(2.2.2)oct-3-yl-4-chlorobenzamide in assays of pain-stimulated and pain-depressed behavior in male Sprague-Dawley rats. Intraperitoneal injection of dilute lactic acid served as an acute noxious stimulus to either stimulate a stretching response or depress the operant responding, which is maintained by electrical brain stimulation in an intracranial self-stimulation (ICSS) procedure. Nicotine produced a dose-dependent, time-dependent, and mecamylamine-reversible blockade of both acid-stimulated stretching and acid-induced depression of ICSS. 5-I-A-85380 also blocked both acid-stimulated stretching and acid-induced depression of ICSS, whereas N-(3R)-1-azabicyclo(2.2.2)oct-3-yl-4-chlorobenzamide produced no effect in either procedure. Both nicotine and 5-I-A-85380 were ≥10-fold more potent in blocking the acid-induced depression of ICSS than in blocking the acid-induced stimulation of stretching. These results suggest that stimulation of α4β2 and/or α6β2 nAChRs may be especially effective to alleviate the signs of pain-related behavioral depression in rats; however, nonselective behavioral effects may contribute to apparent antinociception.

The effects of varenicline on methamphetamine self-administration and drug-primed reinstatement in female rats

While research has revealed heightened vulnerability to meth addiction in women, preclinical models rarely use female subjects when investigating meth seeking and relapse. The goal of the present study was to examine the effects of varenicline (Chantix(®)), a partial α4β2 and full α7 nicotinic acetylcholine receptor agonist, on meth self-administration and reinstatement in female rats. Sprague-Dawley rats were surgically implanted with an indwelling jugular catheter. Half of the rats were then trained to self-administer meth (0.056 mg/kg/infusion) on a variable ratio 3 schedule of reinforcement; the other half earned intravenous saline during daily, 2h sessions. When responding stabilized, varenicline (0.0, 0.3, 1.0, 3.0mg/kg) was tested to determine how it altered meth taking. Varenicline was probed on 4 test days; each test separated by 2 standard self-administration sessions to assure responding remained stable. Following this testing was 15 extinction sessions. Twenty-four hours after the last extinction session were four consecutive days of meth-primed reinstatement. The same 4 doses of varenicline were examined to determine how it altered reinstatement triggered by 0.3mg/kg meth (IP). Rats readily self-administered meth. The higher doses of varenicline did not affect meth-taking in a specific fashion as active lever pressing was also slightly reduced in rats that has access to saline in the self-administration phase. Female rats displayed robust meth-primed reinstatement. Notably, the lower doses of varenicline increased meth-primed reinstatement. This amplified susceptibility to reinstatement (i.e., relapse) may be an impediment for the use of varenicline as a therapeutic to treat meth use disorder.

Existing and Future Drugs for the Treatment of the Dark Side of Addiction

The identification of a heuristic framework for the stages of the addiction cycle that are linked to neurocircuitry changes in pathophysiology includes the binge/intoxication stage, the withdrawal/negative affect stage, and the preoccupation/anticipation (craving) stage, which represent neuroadaptations in three neurocircuits (basal ganglia, extended amygdala, and frontal cortex, respectively). The identification of excellent and validated animal models, the development of human laboratory models, and an enormous surge in our understanding of neurocircuitry and neuropharmacological mechanisms have provided a revisionist view of addiction that emphasizes the loss of brain reward function and gain of stress function that drive negative reinforcement (the dark side of addiction) as a key to compulsive drug seeking. Reversing the dark side of addiction not only explains much of the existing successful pharmacotherapies for addiction but also points to vast new opportunities for future medications to alleviate this major source of human suffering.

Effects of nicotine and minor tobacco alkaloids on intracranial-self-stimulation in rats

BACKGROUND

While nicotine is the primary addictive compound in tobacco, other tobacco constituents including minor alkaloids (e.g., nornicotine, anabasine) may also contribute to tobacco addiction by mimicking or enhancing the effects of nicotine. Further evaluating the behavioral effects of minor alkaloids is essential for understanding their impact on tobacco addiction and informing development of tobacco product standards by the FDA.

METHODS

This study compared the addiction-related effects of nicotine and the minor alkaloids nornicotine, anabasine, myosmine, anatabine, and cotinine on intracranial self-stimulation (ICSS) thresholds in rats.

RESULTS

Acute injection of nicotine produced reinforcement-enhancing (ICSS threshold-decreasing) effects at low to moderate doses, and reinforcement-attenuating/aversive (ICSS threshold-increasing) effects at high doses. Nornicotine and anabasine produced similar biphasic effects on ICSS thresholds, although with lower potency compared to nicotine. Myosmine only elevated ICSS thresholds at relatively high doses, while anatabine and cotinine did not influence ICSS thresholds at any dose. None of the alkaloids significantly influenced ICSS response latencies, indicating a lack of nonspecific motoric effects.

CONCLUSIONS

These findings indicate that some minor tobacco alkaloids can either fully (nornicotine, anabasine) or partially (myosmine) mimic nicotine's addiction-related effects on ICSS, albeit at reduced potency. These findings emphasize the need for further study of the abuse potential of minor alkaloids, including evaluation of their effects when combined with nicotine and other tobacco constituents to better simulate tobacco exposure in humans. Such work is essential for informing FDA regulation of tobacco products and could also lead to the development of novel pharmacotherapies for tobacco addiction.

Examination of the effects of varenicline, bupropion, lorcaserin, or naltrexone on responding for conditioned reinforcement in nicotine-exposed rats

Smoking tobacco remains one of the leading causes of preventable deaths in North America. Nicotine reinforces smoking behavior, in part, by enhancing the reinforcing properties of reward-related stimuli, or conditioned stimuli (CSs), associated with tobacco intake. To investigate how pharmaceutical interventions may affect this property of nicotine, we examined the effect of four US Food and Drug Administration (FDA) approved drugs on the ability of nicotine to enhance operant responding for a CS as a conditioned reinforcer. Thirsty rats were exposed to 13 Pavlovian sessions where a CS was paired with water delivery. Nicotine (0.4 mg/kg) injections were administered before each Pavlovian session. Then, in separate groups of rats, the effects of varenicline (1 mg/kg), bupropion (10 and 30 mg/kg), lorcaserin (0.6 mg/kg), and naltrexone (2 mg/kg), and their interaction with nicotine on responding for conditioned reinforcement were examined. Varenicline and lorcaserin each reduced nicotine-enhanced responding for conditioned reinforcement, whereas naltrexone had a modest effect of reducing response enhancements by nicotine. In contrast, bupropion enhanced the effect of nicotine on this measure. The results of these studies may inform how pharmaceutical interventions can affect smoking cessation attempts and relapse through diverse mechanisms, either substituting for, or interacting with, the reinforcement-enhancing properties of nicotine.

Animal models to assess the abuse liability of tobacco products: effects of smokeless tobacco extracts on intracranial self-stimulation

BACKGROUND

Preclinical models are needed to inform regulation of tobacco products by the Food and Drug Administration (FDA). Typically, animal models of tobacco addiction involve exposure to nicotine alone or nicotine combined with isolated tobacco constituents (e.g. minor alkaloids). The goal of this study was to develop a model using extracts derived from tobacco products that contain a range of tobacco constituents to more closely model product exposure in humans.

METHODS

This study compared the addiction-related effects of nicotine alone and nicotine dose-equivalent concentrations of aqueous smokeless tobacco extracts on intracranial self-stimulation (ICSS) in rats. Extracts were prepared from Kodiak Wintergreen, a conventional product, or Camel Snus, a potential "modified risk tobacco product". Binding affinities of nicotine alone and extracts at various nicotinic acetylcholine receptor (nAChR) subtypes were also compared.

RESULTS

Kodiak and Camel Snus extracts contained levels of minor alkaloids within the range of those shown to enhance nicotine's behavioral effects when studied in isolation. Nonetheless, acute injection of both extracts produced reinforcement-enhancing (ICSS threshold-decreasing) effects similar to those of nicotine alone at low to moderate nicotine doses, as well as similar reinforcement-attenuating/aversive (ICSS threshold-increasing) effects at high nicotine doses. Extracts and nicotine alone also had similar binding affinity at all nAChRs studied.

CONCLUSIONS

Relative nicotine content is the primary pharmacological determinant of the abuse liability of Kodiak and Camel Snus as measured using ICSS. These models may be useful to compare the relative abuse liability of other tobacco products and to model FDA-mandated changes in product performance standards.

The role of mesoaccumbens dopamine in nicotine dependence

There is abundant evidence that the dopamine (DA) neurons that project to the nucleus accumbens play a central role in neurobiological mechanisms underpinning drug dependence. This chapter considers the ways in which these projections facilitate the addiction to nicotine and tobacco. It focuses on the complimentary roles of the two principal subdivisions of the nucleus accumbens, the accumbal core and shell, in the acquisition and maintenance of nicotine-seeking behavior. The ways in which tonic and phasic firing of the neurons contributes to the ways in which the accumbens mediate the behavioral responses to nicotine are also considered. Experimental studies suggest that nicotine has relatively weak addictive properties which are insufficient to explain the powerful addictive properties of tobacco smoke. This chapter discusses hypotheses that seek to explain this conundrum. They implicate both discrete sensory stimuli closely paired with the delivery of tobacco smoke and contextual stimuli habitually associated with the delivery of the drug. The mechanisms by which each type of stimulus influence tobacco dependence are hypothesized to depend upon the increased DA release and overflow, respectively, in the two subdivisions of the accumbens. It is suggested that a majority of pharmacotherapies for tobacco dependence are not more successful because they fail to address this important aspect of the dependence.

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

RATIONALE

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

OBJECTIVES

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Intracranial self-stimulation to evaluate abuse potential of drugs

Intracranial self-stimulation (ICSS) is a behavioral procedure in which operant responding is maintained by pulses of electrical brain stimulation. In research to study abuse-related drug effects, ICSS relies on electrode placements that target the medial forebrain bundle at the level of the lateral hypothalamus, and experimental sessions manipulate frequency or amplitude of stimulation to engender a wide range of baseline response rates or response probabilities. Under these conditions, drug-induced increases in low rates/probabilities of responding maintained by low frequencies/amplitudes of stimulation are interpreted as an abuse-related effect. Conversely, drug-induced decreases in high rates/probabilities of responding maintained by high frequencies/amplitudes of stimulation can be interpreted as an abuse-limiting effect. Overall abuse potential can be inferred from the relative expression of abuse-related and abuse-limiting effects. The sensitivity and selectivity of ICSS to detect abuse potential of many classes of abused drugs is similar to the sensitivity and selectivity of drug self-administration procedures. Moreover, similar to progressive-ratio drug self-administration procedures, ICSS data can be used to rank the relative abuse potential of different drugs. Strengths of ICSS in comparison with drug self-administration include 1) potential for simultaneous evaluation of both abuse-related and abuse-limiting effects, 2) flexibility for use with various routes of drug administration or drug vehicles, 3) utility for studies in drug-naive subjects as well as in subjects with controlled levels of prior drug exposure, and 4) utility for studies of drug time course. Taken together, these considerations suggest that ICSS can make significant contributions to the practice of abuse potential testing.

Single dose varenicline may trigger epileptic activity

Varenicline is a new drug for smoking cessation, and its effect on epilepsy is not clear. The aim of this study was to investigate whether different doses of varenicline cause epileptic activity. Forty rats were randomly assigned to the following eight groups: control, saline, and 0.025, 0.04, 0.1, 0.5, 1, and 2 mg kg(-1) varenicline (single dose, i.p.). EEGs were recorded before the varenicline injection and during the following 240 min. While epileptic discharges were observed on the EEGs of the rats in all of the varenicline-treated groups, motor findings of epileptic seizure were not observed in some rats in these groups except the 1 and 2 mg kg(-1) groups. These findings indicate that different single doses of varenicline cause epileptic activity in rats.

Involvement of neuronal β2 subunit-containing nicotinic acetylcholine receptors in nicotine reward and withdrawal: implications for pharmacotherapies

WHAT IS KNOWN AND OBJECTIVE

Tobacco smoking remains a major health problem. Nicotine binds to nicotinic acetylcholine receptors (nAChRs), which can cause addiction and withdrawal symptoms upon cessation of nicotine administration. Pharmacotherapies for nicotine addiction target brain alterations that underlie withdrawal symptoms. This review will delineate the involvement of the β2 subunit of neuronal nAChRs in nicotine reward and in generating withdrawal symptoms to better understand the efficacy of smoking cessation pharmacotherapies.

COMMENT

Chronic nicotine desensitizes and upregulates β2 subunit-containing nAChRs, and the prolonged upregulation of receptors may underlie symptoms of withdrawal. Experimental research has demonstrated that the β2 subunit of neuronal nAChRs is necessary for generating nicotine reward and withdrawal symptoms.

WHAT IS NEW AND CONCLUSION

Smoking cessation pharmacotherapies act on β2 subunit-containing nAChRs to reduce nicotine reward and withdrawal symptom severity.

Varenicline and cytisine diminish the dysphoric-like state associated with spontaneous nicotine withdrawal in rats

Tobacco addiction is characterized by a negative mood state upon smoking cessation and relapse after periods of abstinence. Clinical studies indicate that negative mood states lead to craving and relapse. The partial α4/α6/β2* nicotinic acetylcholine receptor (nAChR) agonists varenicline and cytisine are widely used as smoking cessation treatments. Varenicline has been approved in the United States for smoking cessation and cytisine is used in Eastern European countries. Despite the widespread use of these compounds, very little is known about their effects on mood states. These studies investigated the effects of varenicline, cytisine, and the cytisine-derivative 3-(pyridin-3'-yl)-cytisine (3-pyr-Cyt) on brain reward function in nicotine-naive and nicotine-withdrawing rats. The cytisine-derivative 3-pyr-Cyt is a very weak α4β2* nAChR partial agonist and like cytisine and varenicline has antidepressant-like effects in animal models. The intracranial self-stimulation (ICSS) procedure was used to investigate the effects of these compounds on brain reward function. Elevations in ICSS thresholds reflect a dysphoric state and a lowering of thresholds is indicative of a potentiation of brain reward function. It was shown that acute administration of nicotine and varenicline lowered ICSS thresholds. Acute administration of cytisine or 3-pyr-Cyt did not affect ICSS thresholds. Discontinuation of chronic, 14 days, nicotine administration led to elevations in ICSS thresholds that lasted for about 2 days. Varenicline and cytisine, but not 3-pyr-Cyt, diminished the nicotine withdrawal-induced elevations in ICSS thresholds. In conclusion, these studies indicate that varenicline and cytisine diminish the dysphoric-like state associated with nicotine withdrawal and may thereby prevent relapse to smoking in humans.

Animal models of nicotine exposure: relevance to second-hand smoking, electronic cigarette use, and compulsive smoking

Much evidence indicates that individuals use tobacco primarily to experience the psychopharmacological properties of nicotine and that a large proportion of smokers eventually become dependent on nicotine. In humans, nicotine acutely produces positive reinforcing effects, including mild euphoria, whereas a nicotine abstinence syndrome with both somatic and affective components is observed after chronic nicotine exposure. Animal models of nicotine self-administration and chronic exposure to nicotine have been critical in unveiling the neurobiological substrates that mediate the acute reinforcing effects of nicotine and emergence of a withdrawal syndrome during abstinence. However, important aspects of the transition from nicotine abuse to nicotine dependence, such as the emergence of increased motivation and compulsive nicotine intake following repeated exposure to the drug, have only recently begun to be modeled in animals. Thus, the neurobiological mechanisms that are involved in these important aspects of nicotine addiction remain largely unknown. In this review, we describe the different animal models available to date and discuss recent advances in animal models of nicotine exposure and nicotine dependence. This review demonstrates that novel animal models of nicotine vapor exposure and escalation of nicotine intake provide a unique opportunity to investigate the neurobiological effects of second-hand nicotine exposure, electronic cigarette use, and the mechanisms that underlie the transition from nicotine use to compulsive nicotine intake.

Effects of the specific α4β2 nAChR antagonist, 2-fluoro-3-(4-nitrophenyl) deschloroepibatidine, on nicotine reward-related behaviors in rats and mice

RATIONALE

Alleviating addiction to tobacco products could prevent millions of deaths. Investigating novel compounds selectively targeting α4β2 nAChRs hypothesized to have a key role in the rewarding effects of nicotine may be a useful approach for future treatment.

OBJECTIVES

The present study was designed to evaluate 2-fluoro-3-(4-nitrophenyl) deschloroepibatidine (4-nitro-PFEB), a potent competitive antagonist of neuronal α4β2 nAChRs, in several animal models related to nicotine reward: drug discrimination, intracranial self-stimulation (ICSS), conditioned place preference, and limited access to self-administration.

METHODS

Long Evans rats were trained in a two-lever discrimination procedure to discriminate 0.4 mg/kg nicotine (s.c.) from saline. Male Sprague-Dawley rats were stereotaxically implanted with electrodes and trained to respond for direct electrical stimulation of the medial forebrain bundle. ICR mice were evaluated using an unbiased place preference paradigm, and finally, male Wistar rats were implanted with intrajugular catheters and tested for nicotine self-administration under limited access (1 h/day).

RESULTS

4-Nitro-PFEB attenuated the discriminative stimulus effects of nicotine, but alone did not produce nicotine-like discriminative stimulus effects. Nicotine-induced facilitation of ICSS reward thresholds was reversed by 4-nitro-PFEB, which alone had no effect on thresholds. 4-Nitro-PFEB also blocked the conditioned place preference produced by nicotine, but alone had no effect on conditioned place preference. Finally, 4-nitro-PFEB dose-dependently decreased nicotine self-administration.

CONCLUSIONS

These results support the hypothesis that neuronal α4β2 nAChRs play a key role in mediating the rewarding effects of nicotine and further suggest that targeting α4β2 nAChRs may yield a potential candidate for the treatment of nicotine dependence.

Delivery of nicotine in an extract of a smokeless tobacco product reduces its reinforcement-attenuating and discriminative stimulus effects in rats

RATIONALE

Animal models of tobacco addiction rely on administration of nicotine alone or nicotine combined with isolated constituents. Models using tobacco extracts derived from tobacco products and containing a range of tobacco constituents might more accurately simulate tobacco exposure in humans.

OBJECTIVE

To compare the effects of nicotine alone and an aqueous smokeless tobacco extract in several addiction-related animal behavioral models.

METHODS

Nicotine alone and nicotine dose-equivalent concentrations of extract were compared in terms of their acute effects on intracranial self-stimulation (ICSS) thresholds, discriminative stimulus effects, and effects on locomotor activity.

RESULTS

Similar levels of nicotine and minor alkaloids were achieved using either artificial saliva or saline for extraction, supporting the clinical relevance of the saline extracts used in these studies. Extract produced reinforcement-enhancing (ICSS threshold-decreasing) effects similar to those of nicotine alone at low to moderate nicotine doses, but reduced reinforcement-attenuating (ICSS threshold-increasing) effects at a high nicotine dose. In rats trained to discriminate nicotine alone from saline, intermediate extract doses did not substitute for the training dose as well as nicotine alone. Locomotor stimulant effects and nicotine distribution to brain were similar following administration of extract or nicotine alone.

CONCLUSIONS

The reinforcement-attenuating and discriminative stimulus effects of nicotine delivered in an extract of a commercial smokeless tobacco product differed from those of nicotine alone. Extracts of tobacco products may be useful for evaluating the abuse liability of those products and understanding the role of non-nicotine constituents in tobacco addiction.

Mouse models for studying genetic influences on factors determining smoking cessation success in humans

Humans differ in their ability to quit using addictive substances, including nicotine, the major psychoactive ingredient in tobacco. For tobacco smoking, a substantial body of evidence, largely derived from twin studies, indicates that approximately half of these individual differences in ability to quit are heritable genetic influences that likely overlap with those for other addictive substances. Both twin and molecular genetic studies support overlapping influences on nicotine addiction vulnerability and smoking cessation success, although there is little formal analysis of the twin data that support this important point. None of the current datasets provides clarity concerning which heritable factors might provide robust dimensions around which individuals differ in ability to quit smoking. One approach to this problem is to test mice with genetic variations in genes that contain human variants that alter quit success. This review considers which features of quit success should be included in a comprehensive approach to elucidate the genetics of quit success, and how those features may be modeled in mice.

Varenicline dose dependently enhances responding for nonpharmacological reinforcers and attenuates the reinforcement-enhancing effects of nicotine

INTRODUCTION

Varenicline (VAR), a partial nicotinic agonist, is one of the most effective smoking cessation pharmacotherapies. The therapeutic efficacy of VAR could be partly the result of substituting for and/or blocking the reinforcement-enhancing effects of nicotine (NIC). We assessed the effects of VAR alone and in combination with NIC (0.4 mg/kg) while rats pressed the lever for a moderately reinforcing visual stimulus (VS).

METHODS

Rats were injected with placebo (0.9% saline), NIC, VAR (0.1-1 mg/kg), or NIC + VAR. A follow-up study was conducted with a broader dose range of VAR-alone dosages (0.01-3.0 mg/kg). All drug manipulations were conducted in a between-subjects design to prevent confounding effects of repeated exposure.

RESULTS

There was a dose-dependent effect of VAR alone. Moderate doses of VAR (0.1 and 1.0 mg/kg) increased the number of VS presentations earned, while lower and higher VAR doses (0.01 and 3.0 mg/kg) did not change responding for the VS. VAR dose dependently attenuated the reinforcement-enhancing effects of NIC, with the highest dose (1.0 mg/kg) exhibiting the greatest antagonist effect.

CONCLUSIONS

The results of these studies support the assertion that the therapeutic efficacy of VAR may be due to the partial agonist characteristics of the drug, specifically, its ability to partially replace the reinforcement-enhancing effects of NIC as well as antagonize these effects.

Neurophysiology of Nicotine Addiction

Tobacco use is a major health problem, and nicotine is the main addictive component. Nicotine binds to nicotinic acetylcholine receptors (nAChR) to produce its initial effects. The nAChRs subtypes are composed of five subunits that can form in numerous combinations with varied functional and pharmacological characteristics. Diverse psychopharmacological effects contribute to the overall process of nicotine addiction, but two general neural systems are emerging as critical for the initiation and maintenance of tobacco use. Mesocorticolimbic circuitry that includes the dopaminergic pathway originating in the ventral tegmental area and projecting to the nucleus accumbens is recognized as vital for reinforcing behaviors during the initiation of nicotine addiction. In this neural system β2, α4, and α6 are the most important nAChR subunits underlying the rewarding aspects of nicotine and nicotine self-administration. On the other hand, the epithalamic habenular complex and the interpeduncular nucleus, which are connected via the fasciculus retroflexus, are critical contributors regulating nicotine dosing and withdrawal symptoms. In this case, the α5 and β4 nAChR subunits have critical roles in combination with other subunits. In both of these neural systems, particular nAChR subtypes have roles that contribute to the overall nicotine addiction process.

Mechanistic insights into nicotine withdrawal

Smoking is responsible for over 400,000 premature deaths in the United States every year, making it the leading cause of preventable death. In addition, smoking-related illness leads to billions of dollars in healthcare expenditures and lost productivity annually. The public is increasingly aware that successfully abstaining from smoking at any age can add years to one's life and reduce many of the harmful effects of smoking. Although the majority of smokers desire to quit, only a small fraction of attempts to quit are actually successful. The symptoms associated with nicotine withdrawal are a primary deterrent to cessation and they need to be quelled to avoid early relapse. This review will focus on the neuroadaptations caused by chronic nicotine exposure and discuss how those changes lead to a withdrawal syndrome upon smoking cessation. Besides examining how nicotine usurps the endogenous reward system, we will discuss how the habenula is part of a circuit that plays a critical role in the aversive effects of high nicotine doses and nicotine withdrawal. We will also provide an updated summary of the role of various nicotinic receptor subtypes in the mechanisms of withdrawal. This growing knowledge provides mechanistic insights into current and future smoking cessation therapies.

Allosteric modulators of the α4β2 subtype of neuronal nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors are ligand-gated ion conducting transmembrane channels from the Cys-loop receptor super-family. The α4β2 subtype is the predominant heteromeric subtype of nicotinic receptors found in the brain. Allosteric modulators for α4β2 receptors interact at a site other than the orthosteric site where acetylcholine binds. Many compounds which act as allosteric modulators of the α4β2 receptors have been identified, with both positive and negative effects. Such allosteric modulators either increase or decrease the response induced by agonist on the α4β2 receptors. Here we discuss the concept of allosterism as it pertains to the α4β2 receptors and summarize the important features of allosteric modulators for this nicotinic receptor subtype.

A lack of association between severity of nicotine withdrawal and individual differences in compensatory nicotine self-administration in rats

RATIONALE

Compensatory smoking may represent an adverse consequence of smoking reduction or the use of reduced-nicotine tobacco products. Factors contributing to individual variability in compensation are poorly understood.

OBJECTIVE

The objective of this study was to examine whether severity of nicotine withdrawal as measured by elevated intracranial self-stimulation (ICSS) thresholds is related to individual differences in compensatory nicotine self-administration (NSA) following unit dose reduction.

METHODS

Rats were trained for ICSS and NSA (0.06 mg/kg per infusion). After stabilization, effects of reducing the nicotine unit dose to 0.03 mg/kg per infusion were examined. Following reacquisition of NSA (0.06 mg/kg per infusion), effects of antagonist-precipitated withdrawal and saline extinction (spontaneous withdrawal) were examined.

RESULTS

Reducing the NSA unit dose produced partial compensation as indicated by the increased infusion rates, but a 35% mean decrease in daily nicotine intake. The magnitude of compensation varied considerably among rats. Dose reduction did not elicit withdrawal in rats as a group, although there were substantial increases in ICSS thresholds in some animals. Intracranial self-stimulation thresholds were consistently elevated during precipitated and spontaneous withdrawal, confirming that rats were nicotine-dependent. Individual differences in compensation were not correlated with changes in ICSS thresholds during dose reduction, precipitated withdrawal, or spontaneous withdrawal. In a secondary analysis, greater precipitated withdrawal severity predicted greater initial nicotine seeking during extinction.

CONCLUSIONS

Severity of nicotine withdrawal was not related to the degree of compensation in this protocol. These data do not support a role for nicotine withdrawal in individual differences in compensation during reduced nicotine exposure, but do suggest that withdrawal may contribute to nicotine seeking during early abstinence.

Varenicline for tobacco dependence: panacea or plight?

INTRODUCTION

This review examines the postmarketing experience with varenicline, including case reports, newer clinical trials and secondary analyses of large clinical datasets.

AREAS COVERED

Varenicline has been shown to be an effective treatment in a broad range of tobacco users with medical, behavioral and diverse demographic characteristics. Recent studies finding excellent safety and efficacy in groups of smokers with diseases including chronic obstructive pulmonary disease are particularly encouraging and call for increased use of this medication for smoking cessation. Despite case reports of serious neuropsychiatric symptoms in patients taking varenicline, including changes in behavior and mood, causality has not been established. Recent analyses of large datasets from clinical trials have not demonstrated that varenicline is associated with more depression or suicidality than other treatments for smoking cessation.

EXPERT OPINION

Now that additional clinical trials in specific populations and observational studies on treatment-seeking smokers outside of clinical trials have been published, we can be confident that varenicline remains the most efficacious monotherapy for smoking cessation and that its side-effect profile remains good. The risk-to-benefit ratio of receiving varenicline to quit smoking must include the increased chances of quitting smoking and avoiding the sizeable risks of smoked-caused disease and death that remain if tobacco addiction is not properly treated.

Addiction and brain reward and antireward pathways

Addictive drugs have in common that they are voluntarily self-administered by laboratory animals (usually avidly), and that they enhance the functioning of the reward circuitry of the brain (producing the 'high' that the drug user seeks). The core reward circuitry consists of an 'in-series' circuit linking the ventral tegmental area, nucleus accumbens and ventral pallidum via the medial forebrain bundle. Although originally believed to simply encode the set point of hedonic tone, these circuits are now believed to be functionally far more complex, also encoding attention, expectancy of reward, disconfirmation of reward expectancy, and incentive motivation. 'Hedonic dysregulation' within these circuits may lead to addiction. The 'second-stage' dopaminergic component in this reward circuitry is the crucial addictive-drug-sensitive component. All addictive drugs have in common that they enhance (directly or indirectly or even transsynaptically) dop-aminergic reward synaptic function in the nucleus accumbens. Drug self-administration is regulated by nucleus accumbens dopamine levels, and is done to keep nucleus accumbens dopamine within a specific elevated range (to maintain a desired hedonic level). For some classes of addictive drugs (e.g. opiates), tolerance to the euphoric effects develops with chronic use. Postuse dysphoria then comes to dominate reward circuit hedonic tone, and addicts no longer use drugs to get high, but simply to get back to normal ('get straight'). The brain circuits mediating the pleasurable effects of addictive drugs are anatomically, neurophysiologically and neurochemically different from those mediating physical dependence, and from those mediating craving and relapse. There are important genetic variations in vulnerability to drug addiction, yet environmental factors such as stress and social defeat also alter brain-reward mechanisms in such a manner as to impart vulnerability to addiction. In short, the 'bio-psycho-social' model of etiology holds very well for addiction. Addiction appears to correlate with a hypodopaminergic dysfunctional state within the reward circuitry of the brain. Neuroimaging studies in humans add credence to this hypothesis. Credible evidence also implicates serotonergic, opioid, endocannabinoid, GABAergic and glutamatergic mechanisms in addiction. Critically, drug addiction progresses from occasional recreational use to impulsive use to habitual compulsive use. This correlates with a progression from reward-driven to habit-driven drug-seeking behavior. This behavioral progression correlates with a neuroanatomical progression from ventral striatal (nucleus accumbens) to dorsal striatal control over drug-seeking behavior. The three classical sets of craving and relapse triggers are (a) reexposure to addictive drugs, (b) stress, and (c) reexposure to environmental cues (people, places, things) previously associated with drug-taking behavior. Drug-triggered relapse involves the nucleus accumbens and the neurotransmitter dopamine. Stress-triggered relapse involves (a) the central nucleus of the amygdala, the bed nucleus of the stria terminalis, and the neurotransmitter corticotrophin-releasing factor, and (b) the lateral tegmental noradrenergic nuclei of the brain stem and the neurotransmitter norepinephrine. Cue-triggered relapse involves the basolateral nucleus of the amygdala, the hippocampus and the neurotransmitter glutamate. Knowledge of the neuroanatomy, neurophysiology, neurochemistry and neuropharmacology of addictive drug action in the brain is currently producing a variety of strategies for pharmacotherapeutic treatment of drug addiction, some of which appear promising.

Translational research in addiction: toward a framework for the development of novel therapeutics

The development of novel substance use disorder (SUD) therapeutics is insufficient to meet the medical needs of a growing SUD patient population. The identification of translatable SUD models and tests is a crucial step in establishing a framework for SUD therapeutic development programs. The present review begins by identifying the clinical features of SUDs and highlights the narrow regulatory end-point required for approval of a novel SUD therapeutic. A conceptual overview of dependence is provided, followed by identification of potential intervention targets in the addiction cycle. The main components of the addiction cycle provide the framework for a discussion of preclinical models and their clinical analogs, all of which are focused on isolated behavioral end-points thought to be relevant to the persistence of compulsive drug use. Thus, the greatest obstacle to successful development is the gap between the multiplicity of preclinical and early clinical end-points and the regulatory end-point of sustained abstinence. This review proposes two pathways to bridging this gap: further development and validation of the preclinical extended access self-administration model; inclusion of secondary end-points comprising all of the measures highlighted in the present discussion in Phase 3 trials. Further, completion of the postdictive validation of analogous preclinical and clinical assays is of high priority. Ultimately, demonstration of the relevance and validity of a variety of end-points to the ultimate goal of abstinence will allow researchers to identify truly relevant therapeutic mechanisms and intervention targets, and establish a framework for SUD therapeutic development that allows optimal decision-making and resource allocation.

The high-affinity nAChR partial agonists varenicline and sazetidine-A exhibit reinforcing properties in rats

Varenicline (Chantix®, Champix®) is a nicotinic acetylcholine receptor (nAChR) partial agonist clinically approved for smoking cessation, yet its potential abuse liability properties have not been fully characterized. The nAChR ligand sazetidine-A has been reported as a selective full or partial agonist at α4β2* nAChR subtypes in in vitro studies. In the present studies, varenicline, sazetidine-A and nicotine exhibited inverted U-shaped dose-response functions under fixed-ratio (peak responding at 30, 60 and 10-30 μg/kg/inf, respectively) or progressive-ratio (peak responding at 30-60, 30-100 and 30 μg/kg/inf, respectively) schedules in rats trained to self-administer nicotine. Varenicline (ED(50) 0.2 mg/kg) and sazetidine-A (ED(50) 0.44 mg/kg) fully substituted for nicotine (ED(50) 0.09 mg/kg) in rats trained to discriminate nicotine (0.4 mg/kg, i.p.) from saline. The reinforcing and discriminative stimulus (DS) properties of sazetidine-A, varenicline and nicotine were attenuated by acute pretreatment with the non-selective neuronal non-competitive nAChR antagonist mecamylamine or the α4* nAChR-selective antagonist dihydro-β-erythroidine, but not by the α7 nAChR subtype antagonist methyllycaconitine. Drug-naïve rats acquired stable self-administration of varenicline (30 μg/kg/inf), and sazetidine-A (60 μg/kg/inf), at doses that supported peak responding under a fixed-ratio 3 schedule in nicotine-trained rats. Nonetheless, self-administration and re-acquisition of varenicline and sazetidine-A were less robust than nicotine. Thus, partial activation of α4β2* nAChRs by varenicline or sazetidine-A is sufficient to mimic the DS and reinforcing properties of nicotine in nicotine-experienced rats, although the reinforcing properties of partial agonists are diminished in nicotine-naïve rats. Future studies should assess nicotine withdrawal measures in animals chronically exposed to varenicline or sazetidine-A.

Preclinical Pharmacology, Efficacy and Safety of Varenicline in Smoking Cessation and Clinical Utility in High Risk Patients

Smoking is still the most prominent cause of preventable premature death in the United States and an increasing cause of morbidity and mortality throughout the world. Although the current treatments such as nicotine replacement therapy (NRT) and bupropion are effective, long-term abstinence rates are low. Mechanism studies suggest that the pleasurable effects of smoking are mediated predominantly by nicotine, which activates the brain reward system by activation of brain α4β2 nicotinic acetylcholine receptors (nAChRs). Varenicline is a novel α4β2 nAChR partial agonist and has been found to be even more effective than NRT or bupropion in attenuating smoking satisfaction and in relieving craving and withdrawal symptoms after abstinence. Thus, varenicline has been recently approved to be a first-line medication for smoking cessation in the United States and European countries. Varenicline is generally well tolerated in healthy adult smokers, with the most commonly reported adverse effects being nausea, insomnia, and headache. However, growing postmarketing data has linked varenicline to an increase in neuropsychiatric symptoms such as seizures, suicidal attempts, depression, and psychosis as well as serious injuries potentially relating to unconsciousness, dizziness, visual disturbances, or movement disorders. Therefore, new safety warnings are issued to certain high risk populations, such as patients with mental illness and operators of commercial vehicles and heavy machinery. In particular, pilots, air traffic controllers, truck and bus drivers have been banned from taking varenicline.

Anti-relapse medications: preclinical models for drug addiction treatment

Addiction is a chronic relapsing brain disease and treatment of relapse to drug-seeking is considered the most challenging part of treating addictive disorders. Relapse can be modeled in laboratory animals using reinstatement paradigms, whereby behavioral responding for a drug is extinguished and then reinstated by different trigger factors, such as environmental cues or stress. In this review, we first describe currently used animal models of relapse, different relapse triggering factors, and the validity of this model to assess relapse in humans. We further summarize the growing body of pharmacological interventions that have shown some promise in treating relapse to psychostimulant addiction. Moreover, we present an overview on the drugs tested in cocaine or methamphetamine addicts and examine the overlap of existing preclinical and clinical data. Finally, based on recent advances in our understanding of the neurobiology of relapse and published preclinical data, we highlight the most promising areas for future anti-relapse medication development.

Mechanism-based medication development for the treatment of nicotine dependence

Tobacco use is a global problem with serious health consequences. Though some treatment options exist, there remains a great need for new effective pharmacotherapies to aid smokers in maintaining long-term abstinence. In the present article, we first discuss the neural mechanisms underlying nicotine reward, and then review various mechanism-based pharmacological agents for the treatment of nicotine dependence. An oversimplified hypothesis of addiction to tobacco is that nicotine is the major addictive component of tobacco. Nicotine binds to alpha4beta2 and alpha7 nicotinic acetylcholine receptors (nAChRs) located on dopaminergic, glutamatergic and GABAergic neurons in the mesolimbic dopamine (DA) system, which causes an increase in extracellular DA in the nucleus accumbens (NAc). That increase in DA reinforces tobacco use, particularly during the acquisition phase. Enhanced glutamate transmission to DA neurons in the ventral tegmental area appears to play an important role in this process. In addition, chronic nicotine treatment increases endocannabinoid levels in the mesolimbic DA system, which indirectly modulates NAc DA release and nicotine reward. Accordingly, pharmacological agents that target brain acetylcholine, DA, glutamate, GABA, or endocannabonoid signaling systems have been proposed to interrupt nicotine action. Furthermore, pharmacokinetic strategies that alter plasma nicotine availability, metabolism and clearance also significantly alter nicotine's action in the brain. Progress using these pharmacodynamic and pharmacokinetic agents is reviewed. For drugs in each category, we discuss the mechanistic rationale for their potential anti-nicotine efficacy, major findings in preclinical and clinical studies, and future research directions.