Effects of varenicline on operant self-administration of alcohol and/or nicotine in a rat model of co-abuse

Varenicline Mitigates the Increased Risk of Pseudoarthrosis Associated with Nicotine

BACKGROUND CONTEXT

High serum nicotine levels increase the risk of non-union after spinal fusion. Varenicline, a pharmaceutical adjunct for smoking cessation, is a partial agonist designed to displace and outcompete nicotine at its receptor binding site, thereby limiting downstream activation. Given its mechanism, varenicline may have therapeutic benefits in mitigating non-union for active smokers undergoing spinal fusion.

PURPOSE

To compare fusion rate and fusion mass characteristics between cohorts receiving nicotine, varenicline, or concurrent nicotine and varenicline after lumbar fusion.

STUDY DESIGN

Rodent non-instrumented spinal fusion model.

METHODS

Sixty eight-week-old male Sprague-Dawley rats weighing approximately 300 grams underwent L4-5 posterolateral fusion (PLF) surgery. Four experimental groups (control: C, nicotine: N, varenicline: V, and combined: NV [nicotine and varenicline]) were included for analysis. Treatment groups received nicotine, varenicline, or a combination of nicotine and varenicline delivered through subcutaneous osmotic pumps beginning two weeks before surgery until the time of sacrifice at age 14 weeks. Manual palpation testing, microCT imaging, bone histomorphometry, and biomechanical testing were performed on harvested spinal fusion segments.

RESULTS

Control (p=0.016) and combined (p=0.032) groups, when compared directly to the nicotine group, demonstrated significantly greater manual palpation scores. The fusion rate in the control (93.3%) and combined (93.3%) groups were significantly greater than that of the nicotine group (33.3%) (p=0.007, both). Biomechanical testing demonstrated greater Young's modulus of the fusion segment in the control (17.1 MPa) and combined groups (34.5 MPa) compared to the nicotine group (8.07 MPa) (p<0.001, both). MicroCT analysis demonstrated greater bone volume fraction (C:0.35 vs N:0.26 vs NV:0.33) (p<0.001, all) and bone mineral density (C:335 vs N:262 vs NV:328 mg Ha/cm³) (p<0.001, all) in the control and combined groups compared to the nicotine group. Histomorphometry demonstrated a greater mineral apposition rate in the combined group compared to the nicotine group (0.34 vs 0.24 μm/day, p=0.025).

CONCLUSION

In a rodent spinal fusion model, varenicline mitigates the adverse effects of high nicotine serum levels on the rate and quality of spinal fusion.

CLINICAL SIGNIFICANCE

These findings have the potential to significantly impact clinical practice guidelines and the use of pharmacotherapy for active nicotine users undergoing fusion surgery.

Differential Effects of Nicotine, Alcohol, and Coexposure on Neuroimmune-Related Protein and Gene Expression in Corticolimbic Brain Regions of Rats

Nicotine and alcohol co-use is extremely common and their use constitutes two of the most common causes of preventable death, yet the underlying biological mechanisms are largely understudied. Activation of neuroimmune toll-like receptors (TLRs) promotes the induction of proinflammatory cascades and increases alcohol intake in rodents, which further promotes TLRs in the brain; nicotine may decrease central proinflammatory signaling. The current studies sought to determine the effects of nicotine ± alcohol (alone or in combination) on circulating blood plasma and TLR protein/gene expression in addiction-associated corticolimbic brain regions, including the prefrontal cortex-prelimbic (mPFC-PL) and nucleus accumbens core (AcbC). Adult rats were treated with alcohol (0 or 2 g/kg, IG) and exposed to nicotine vapor (0 or 30 mg/mL solution) daily for 2, 14, or 28 days. Plasma studies indicated no effects of independent exposure or coexposure in males. Coexposure decreased plasma nicotine levels versus nicotine-only treated females, yet alcohol and cotinine concentrations were unchanged. By 28 days, the anti-inflammatory cytokine IL-13 was decreased in alcohol-only females. Divergent changes in TLR3 (but not TLR4) protein occurred for independent-drug exposed males (but not coexposure), with reductions in the mPFC-PL after 14 days and increases in the AcbC by 28 days. Gene expression following chronic coexposure suggests nicotine may regionally counteract alcohol-induced inflammation, including increased AcbC-TLR3/4/7 and several downstream markers in females and increased mPFC-PL-TLR3 and -STAT3 (but not IRF3) evident in males with exposure to either drug alone. These findings give further insight into the role of sex and the neuroimmune system in independent exposure and coexposure to nicotine ± alcohol.

Neuropharmacology of Alcohol Addiction with Special Emphasis on Proteomic Approaches for Identification of Novel Therapeutic Targets

Alcohol is a generic pharmacological agent with only a few recognized primary targets. Nmethyl- D-aspartate, gamma-aminobutyric acid, glycine, 5-hydroxytryptamine 3 (serotonin), nicotinic acetylcholine receptors, and L-type Ca2+ channels and G-protein-activated inwardly rectifying K channels are all involved. Following the first hit of alcohol on specific brain targets, the second wave of indirect effects on various neurotransmitter/neuropeptide systems begins, leading to the typical acute behavioral effects of alcohol, which range from disinhibition to sedation and even hypnosis as alcohol concentrations rise. Recent research has revealed that gene regulation is significantly more complex than previously thought and does not fully explain changes in protein levels. As a result, studying the proteome directly, which differs from the genome/transcriptome in terms of complexity and dynamicity, has provided unique insights into extraordinary advances in proteomic techniques that have changed the way we can analyze the composition, regulation, and function of protein complexes and pathways underlying altered neurobiological conditions. Neuroproteomics has the potential to revolutionize alcohol research by allowing researchers to gain a better knowledge of how alcohol impacts protein structure, function, connections, and networks on a global scale. The amount of information collected from these breakthroughs can aid in identifying valuable biomarkers for early detection and improved prognosis of an alcohol use disorder and future pharmaceutical targets for the treatment of alcoholism.

The incentive amplifying effects of nicotine: Roles in alcohol seeking and consumption

Nicotine has a unique profile among drugs of abuse. To the noninitiated user, nicotine has powerful aversive effects and its relatively weak euphorigenic effects undergo rapid tolerance. Despite this, nicotine is commonly abused despite negative heath consequences, and nicotine users have enormous difficulty quitting. Further, nicotine is one of the most commonly co-abused substances, in that it is often taken in combination with other drugs. One explanation of this polydrug use is that nicotine has multiple appetitive and consummatory conditioning effects. For example, nicotine is a reinforcement enhancer in that it can potently increase the incentive value of other stimuli, including those surrounding drugs of abuse such as alcohol. In addition, nicotine also has a unique profile of neurobiological effects that alter regulation of alcohol intake and interoception. This review discusses the psychological and biological mechanisms surrounding nicotine's appetitive conditioning and consummatory effects, particularly its interactions with alcohol.

Combined smoking and alcohol cues: Effects on craving, drug-seeking, and consumption

BACKGROUND

Alcohol and cigarettes are commonly used together, but little is known about their joint motivational impact. Cue reactivity studies have customarily examined alcohol and smoking cues in isolation, despite the potential for cues to elicit stronger motivational responses when combined. This study used a validated cue reactivity procedure (Choice Behavior Under Cued Conditions) systematically to disentangle the separate and joint effects of alcohol and cigarette cues on substance use motivation.

METHODS

Participants were 110 adults (M_age  = 34.0, SD = 10.8) who consumed both cigarettes and alcohol. Participants completed 40 cue reactivity trials with four in vivo cue types: water, alcohol, cigarette, and combined cigarette and alcohol. Participants rated their craving prior to receiving opportunities to spend real money to gain access to the cues. Spending larger amounts of money increased the probability that the substance(s) would be available for consumption. When granted access, participants took one cigarette puff and/or sip of the beverage. A multimethod approach assessed three key motivational indices: craving, drug-seeking (spending, latency to access the cue), and consumption (puff duration, alcohol consumed). Effects of cue type and rates of substance use (cigarettes per day, drinks per day, relative frequency of co-use) were assessed using hierarchical linear models.

RESULTS

Both alcohol and smoking cues enhanced cue-specific craving but not craving for the alternative substance. In a novel finding, combined cues elicited higher craving and greater spending than single-drug cues. All drug cues elicited greater spending than water cues, and spending was moderated by the relative frequency of co-use.

CONCLUSIONS

We found that combined alcohol and cigarette cues provoke more powerful craving and drug-seeking responses and, therefore, may be more motivationally potent among individuals who use multiple substances.

Targeting the Opioid Receptors: A Promising Therapeutic Avenue for Treatment in “Heavy Drinking Smokers”

Aims

Despite a general decline in tobacco use in the last decades, the prevalence of tobacco smoking in individuals with alcohol use disorder (AUD) remains substantial (45–50%). Importantly, the co-use of both substances potentiates the adverse effects, making it a significant public health problem. Substantial evidence suggests that AUD and Tobacco use disorder (TUD) may share common mechanisms. Targeting these mechanisms may therefore provide more effective therapy. Numerous studies describe a potential role of the endogenous opioid system in both AUD and TUD. Reviewing this literature, we aim to evaluate the efficacy of molecules that target the opioid system as promising therapeutic interventions for treating alcohol and tobacco co-use disorders.

Methods

We provide a synthesis of the current epidemiological knowledge of alcohol and tobacco co-use disorders. We evaluate clinical and preclinical research that focuses on the regulation of the endogenous opioid system in alcohol, nicotine, and their interactions.

Results

The epidemiological data confirm that smoking stimulates heavy drinking and facilitates alcohol craving. Pharmacological findings suggest that treatments that are efficacious in the dual addiction provide a beneficial treatment outcome in comorbid AUD and TUD. In this regard, MOP, DOP and NOP-receptor antagonists show promising results, while the findings prompt caution when considering KOP-receptor antagonists as a treatment option in alcohol and tobacco co-use disorders.

Conclusions

Existing literature suggests a role of the opioid system in sustaining the high comorbidity rates of AUD and TUD. Molecules targeting opioid receptors may therefore represent promising therapeutic interventions in ‘heavy drinking smokers.’

The role of nicotinic acetylcholine receptors in alcohol-related behaviors

Alcohol use disorder (AUD) causes an alarming economic and health burden in the United States. Unfortunately, this disease does not exist in isolation; AUD is highly comorbid with nicotine use. Results from both human and animal models demonstrate a genetic correlation between alcohol and nicotine behaviors. These data support the idea of shared genetic and neural mechanisms underlying these behaviors. Nicotine acts directly at nicotinic acetylcholine receptors (nAChR) to have its pharmacological effect. Interestingly, alcohol also acts both directly and indirectly at these receptors. Research utilizing genetically engineered rodents and pharmacological manipulations suggest a role for nAChR in several ethanol behaviors. The current manuscript collates this literature and discusses findings that implicate specific nAChR subunits in ethanol phenotypes. These data suggest future directions for targeting nAChR as novel therapeutics for AUD.

Effects of ethanol, naltrexone, nicotine and varenicline in an ethanol and nicotine co-use model in Sprague-Dawley rats

BACKGROUND

As alcohol and nicotine use disorders are entwined, it may be possible to develop a single medication to treat both. We previously developed a model for ethanol (EtOH) and nicotine co-use in female selectively bred alcohol-preferring (P) rats. To model co-use in a genetically diverse population, we adapted the model to outbred Sprague-Dawley rats of both sexes and assessed the effect of drug pretreatments.

METHODS

In phase 1, rats were trained in a 2-bottle choice between water and a sweetened or unsweetened EtOH solution in operant chambers. In phase 2, rats were trained in nicotine self-administration under an increasing fixed ratio (FR) schedule with 2 bottles containing water or saccharin-sweetened EtOH also available. In phase 3, rats were pretreated with EtOH (0.5, 1.5 g/kg), naltrexone (0.3 mg/kg), nicotine (0.2, 0.6 mg/kg), varenicline (3.0 mg/kg) or vehicle before the session.

RESULTS

Sweetening the EtOH solution was required to obtain pharmacologically relevant levels of consumption in Phase 1, with males showing increased sweetened EtOH preference compared to females. In Phase 2, increasing the FR requirement for nicotine decreased nicotine infusions, but increased EtOH consumption. In Phase 3, EtOH, naltrexone, and nicotine failed to alter EtOH consumption; however, varenicline decreased both EtOH and nicotine intake.

CONCLUSIONS

The co-use model was successfully adapted to Sprague-Dawley rats by adding saccharin to the EtOH solution. In contrast to previous results in P rats, varenicline reduced both EtOH and nicotine intake, indicating it may be a useful monotherapy for co-use in a genetically diverse population.

Assessment of individual differences in response to acute bupropion or varenicline treatment using a long-access nicotine self-administration model and behavioral economics in female rats

Bupropion and varenicline are widely prescribed pharmacological treatments for smoking cessation. These treatments are only marginally effective in clinical populations but most preclinical studies show that they are effective in decreasing self-administration in rats on a group level. The present study investigated individual differences in responding to bupropion or varenicline in a preclinical model of long-access to nicotine (0.03 mg/kg/inf; 12 h/day) in female rats. Rats were first assessed for their individual economic demand for nicotine and for their individual performance in open field and elevated plus maze prior to nicotine access and during withdrawal. Rats were then tested for the acute effects of bupropion, varenicline, and yohimbine. We found that neither bupropion nor varenicline decreased responding for nicotine on test days. On the contrary, a moderate dose of bupropion (30 mg/kg) significantly increased responding for nicotine. We also found that rats with higher demand for nicotine were more sensitive to pretreatment with yohimbine which resulted in increased responding for nicotine during the dose-effect tests. Finally, we show that rats that had a higher demand for nicotine also were more persistent in seeking nicotine during extinction and reinstatement tests with nicotine or yohimbine as triggers. Our findings suggest that the length of access to daily nicotine may be an important factor underlying the response to pharmacological treatments like bupropion or varenicline. Future studies modeling chronic treatment approaches that include both sexes will be needed to further extend our findings.

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.

Neurochemical Evidence of Preclinical and Clinical Reports on Target-Based Therapy in Alcohol Used Disorder

Alcohol use disorder (AUD) is a chronic relapsing disorder, which enforces a person to compulsively seek alcohol, restricting control over alcohol intake leads to emergence of an undesired emotional state during abstinence. There are recent advances for better understanding of neurocircuitry involved in the pathophysiology of AUD. Alcohol interaction with neuronal membrane proteins results in changes in neuronal circuits. It is also linked with the potential medication and their clinical validation concerning their pharmacological targets for alcoholic abstinence. This review covers research work from the past few decades on the therapeutic advances on treatment of alcohol dependence; further detailing the fundamental neurochemical mechanisms after alcohol administration. It also covers interaction of alcohol with GABAergic, glutaminergic, dopaminergic, serotonergic and opioid systems. This review further elaborated the neurobiology of noradrenergic, cholinergic and cannabinoid systems and their interaction with AUD. Elaborative information of potential drug targets under current exploration for AUD treatment with their mechanisms are reported here along with clinical outcomes and the associated side effects.

Development of a novel alcohol and nicotine concurrent access (ANCA) self-administration procedure in baboons

BACKGROUND

Self-administration of either alcohol or nicotine under single-access conditions has been studied extensively in laboratory animals. Relatively few studies have examined the co-use of these substances, even though alcohol and nicotine use and abuse commonly co-occur in humans. The objectives of this study were to develop a baboon model of concurrent alcohol and nicotine self-administration, and examine effects of varenicline on alcohol and nicotine co-use.

METHODS

In Experiment 1, five male baboons were trained to self-administer drinks of alcohol (4% w/v) and injections of nicotine (0.032-0.1 mg/kg) under single-access and then concurrent-access conditions, and intake of alcohol (g/kg) and nicotine (mg/kg) was compared under single- and concurrent-access conditions. In Experiment 2, three male baboons self-administered drinks of alcohol (4% w/v) and injections of nicotine (0.056 mg/kg) under concurrent-access conditions. Pretreatment with varenicline (0.32-1.0 mg/kg, s.c.) or an equal volume of its vehicle before concurrent-access sessions was repeated for 5 consecutive days.

RESULTS

Self-administration of nicotine and alcohol was successfully established under both single- and concurrent-access conditions that produced reliable levels of voluntary alcohol and nicotine intake. Co-self-administration of both drugs produced levels of intake similar to that produced by each drug alone. Varenicline significantly reduced intake of both alcohol and nicotine when compared to the vehicle condition.

CONCLUSIONS

This baboon model provides a valuable tool for further investigation of the behavioral and pharmacological mechanisms involved in co-use of nicotine and alcohol. A single pharmacotherapeutic agent (e.g., varenicline) may be useful in treating nicotine and alcohol co-use.

Varenicline Targets the Reinforcing-Enhancing Effect of Nicotine on Its Associated Salient Cue During Nicotine Self-administration in the Rat

Nicotine is acknowledged as the key addictive compound of tobacco. Varenicline (Champix^® or Chantix^®), mainly acting as a partial agonist at the α4β2 nicotinic receptor, is an approved smoking cessation pharmacotherapy, although with efficacy limited to a portion of smokers. Smokers differ in the motives that drive their drug seeking and Varenicline might be more efficient in some groups more than others. Studies in rodents revealed that nicotine-seeking is strongly supported by complex interactions between nicotine and environmental cues, and notably the ability of nicotine to enhance the reinforcing properties of salient environmental stimuli. It is not yet understood whether the decrease of nicotine-seeking by acute Varenicline in rats results from antagonism of the primary reinforcing effects of nicotine, of the reinforcement-enhancing effect of nicotine on cues, or of a combination of both. Thanks to a protocol that allows assessment of the reinforcement-enhancing effect of nicotine on cues during self-administration in rats, we showed that Varenicline targets both nicotine reinforcing effects and reinforcement-enhancing effect of nicotine on cues. Importantly, individual variations in the latter determined the amplitude of acute Varenicline-induced decrease in seeking. These results suggest that Varenicline might be more beneficial in smokers who are more sensitive to nicotine effects on surrounding stimuli.

Therapeutic challenges for concurrent ethanol and nicotine consumption: naltrexone and varenicline fail to alter simultaneous ethanol and nicotine intake by female alcohol-preferring (P) rats

RATIONALE AND OBJECTIVES

Simultaneous alcohol and nicotine consumption occurs in the majority of individuals with alcohol use disorder (AUD) and nicotine dependence. Varenicline (Var) is used to assist in the cessation of nicotine use, while naltrexone (Nal) is the standard treatment for AUD. Despite evidence that ethanol (EtOH) and nicotine (NIC) co-use produces unique neuroadaptations, preclinical research has focused on the effects of pharmacotherapeutics on a single reinforcer. The current experiments examined the effects of Var and Nal on EtOH, NIC, or EtOH+NIC intake.

METHODS

Animals were randomly assigned to one of four drinking conditions of 24-h access to a three-bottle choice paradigm, one of which always contained water. Drinking conditions were water only, 0.07 and 0.14 mg/mL NIC (NIC only), 15% and 30% EtOH (EtOH only), or 15% and 30% EtOH with 0.14 mg/mL NIC (EtOH+NIC). The effects of Var (0, 1, or 2 mg/kg) or Nal (0, 1, or 10 mg/kg) injections on maintenance and relapse consumption were determined during four consecutive days.

RESULTS

Var reduced maintenance and relapse NIC intake but had no effect on EtOH or EtOH+NIC drinking. Conversely, Nal reduced EtOH maintenance and relapse drinking, but had no effect on NIC or EtOH+NIC drinking.

DISCUSSION

The results indicate the standard pharmacological treatments for nicotine dependence and AUD were effective at reducing consumption of the targeted reinforcer but neither reduced EtOH+NIC co-use/abuse. These findings suggest that co-abuse may promote unique neuroadaptations that require models of polysubstance abuse to develop pharmacotherapeutics to treat AUD and nicotine dependence.

Preclinical evidence for combining the 5-HT2C receptor agonist lorcaserin and varenicline as a treatment for nicotine dependence

Varenicline, a nicotinic acetylcholine receptor partial agonist, is used to treat nicotine dependence. Lorcaserin, a 5-HT_2C receptor agonist has been approved in some countries to treat obesity. Based on preclinical and preliminary clinical evidence, lorcaserin may have potential to treat nicotine dependence. These experiments examined in rats the effects of combining varenicline (0.5 or 1 mg/kg) and lorcaserin (0.3, 0.6 and 1 mg/kg) on nicotine self-administration, reinstatement of nicotine seeking, responding for food and impulsive action. Both drugs alone reduced nicotine self-administration. Combining varenicline and 0.6 mg/kg lorcaserin reduced responding to a greater extent than either drug alone. In a relapse model, extinguished nicotine seeking was reinstated by a priming injection of nicotine and nicotine-associated cues. Reinstatement was reduced by varenicline (1 mg/kg) and by lorcaserin (0.3 mg/kg). Combining lorcaserin (0.3 mg/kg) with varenicline (0.5 or 1 mg/kg) reduced reinstatement to a greater degree than either drug alone. Both drugs had minimal effects on responding for food, alone or in combination. In the five-choice serial reaction time test, varenicline (0.5 or 1 mg/kg) increased impulsivity, measured as increased premature responding. This effect was reduced by lorcaserin (0.3 mg/kg). Plasma levels of varenicline or lorcaserin were not altered by co-administration of the other drug. Varenicline and lorcaserin have additive effects on nicotine self-administration, and on nicotine seeking. Lorcaserin prevents impulsivity induced by varenicline. This pattern of effects suggests that co-administration of varenicline and lorcaserin has potential as a treatment for nicotine dependence that may exceed the value of either drug alone.

Individual differences in responding to bupropion or varenicline in a preclinical model of nicotine self-administration vary according to individual demand for nicotine

Bupropion and varenicline are the top two smoking cessation interventions that are marginally successful in increasing abstinence rates when compared to placebo. Although smokers vary in their history and pattern of tobacco use, there is a significant gap in addressing this individual variability with individually targeted treatments. The present study takes the initial step towards a better understanding of individual differences in treatment outcomes by assessing the effect of bupropion or varenicline on nicotine self-administration in rats. Rats were first assessed for their individual economic demand for sucrose and then for self-administered nicotine (0.03 mg/kg/inf; 2 h sessions). We then examined the effect of bupropion (0, 10, 30, 60 mg/kg) or varenicline (0, 0.1, 1.0, 3.0 mg/kg) pretreatment on individual rates of nicotine self-administration using progressive ratio schedule of reinforcement. Thereafter, rats were subjected to four rounds of extinction and reinstatement tests. We found that individual demand for sucrose did not predict individual demand for nicotine. Acute pretreatments with bupropion or varenicline were most effective at decreasing nicotine self-administration in rats that had a higher demand for nicotine. Rats with higher demand for nicotine also showed higher magnitude of responding in extinction and during nicotine-triggered reinstatement tests. Although the acute treatment protocol employed in this study is an important initial step towards a better understanding of individual treatment effects, future research modeling chronic treatment approaches will be needed to further extend our findings.

Nicotine Self-Administration as Paradigm for Medication Discovery for Smoking Cessation: Recent Findings in Medications Targeting the Cholinergic System

Tobacco kills every year approximately six million people as a direct result of direct use, and it is still considered one of the most excruciating threats for human health worldwide. The low successful rates of the currently available pharmacotherapies to assist in quitting tobacco use suggest there is a need for more effective treatments.The intravenous self-administration (IVSA) paradigm is considered the gold standard to study voluntary drug intake in animal models, including nicotine. The IVSA paradigm has been used to identify key mechanisms involved in the addictive properties of nicotine in both rodents and nonhuman primates. In this chapter we describe how the IVSA paradigm has served to further investigate the role of nicotinic acetylcholine receptors (nAChRs) in the reinforcing properties of nicotine. Notably, this review will cover recent advances (i.e., research carried out during the past decade) using the IVSA paradigm, with a focus on the status of research on current smoking cessation medications (such as varenicline and bupropion) and of other nAChR ligands.The combination of the IVSA paradigm with pharmacological and genetic tools (e.g., knockout animals) has greatly contributed to our understanding of the role of specific subtype nAChRs in nicotine reinforcement processes. We also discuss some of the limitations of the IVSA paradigm so these can be taken into consideration when interpreting and designing new studies.

An improved model of ethanol and nicotine co-use in female P rats: Effects of naltrexone, varenicline, and the selective nicotinic α6β2* antagonist r-bPiDI

Background Although pharmacotherapies are available for alcohol (EtOH) or tobacco use disorders individually, it may be possible to develop a single pharmacotherapy to treat heavy drinking tobacco smokers by capitalizing on the commonalities in their mechanisms of action. Methods Female alcohol-preferring (P) rats were trained for EtOH drinking and nicotine self-administration in two phases: (1) EtOH alone (0 vs. 15% EtOH, 2-bottle choice) and (2) concomitant access, during which EtOH access continued with access to nicotine (0.03 mg/kg/infusion, i.v.) using a 2-lever choice procedure (active vs. inactive lever) in which the fixed ratio (FR) requirement was gradually increased to FR30. When stable co-use was obtained, rats were pretreated with varying doses of naltrexone, varenicline, or r-bPiDI, an α6β2* subtype-selective nicotinic acetylcholine receptor antagonist shown previously to reduce nicotine self-administration. Results While EtOH intake was initially suppressed in phase 2 (co-use), pharmacologically relevant intake for both substances was achieved by raising the "price" of nicotine to FR30. In phase 2, naltrexone decreased EtOH and water consumption but not nicotine intake; in contrast, naltrexone in phase 1 (EtOH only) did not significantly alter EtOH intake. Varenicline and r-bPiDI in phase 2 both decreased nicotine self-administration and inactive lever pressing, but neither altered EtOH or water consumption. Conclusions These results indicate that increasing the "price" of nicotine increases EtOH intake during co-use. Additionally, the efficacy of naltrexone, varenicline, and r-bPiDI was specific to either EtOH or nicotine, with no efficacy for co-use. Nevertheless, future studies on combining these treatments may reveal synergistic efficacy.

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 the nicotinic agonist varenicline, nicotinic antagonist r-bPiDI, and DAT inhibitor (R)-modafinil on co-use of ethanol and nicotine in female P rats

RATIONALE

Co-users of alcohol and nicotine are the largest group of polysubstance users worldwide. Commonalities in mechanisms of action for ethanol (EtOH) and nicotine proposes the possibility of developing a single pharmacotherapeutic to treat co-use.

OBJECTIVES

Toward developing a preclinical model of co-use, female alcohol-preferring (P) rats were trained for voluntary EtOH drinking and i.v. nicotine self-administration in three phases: (1) EtOH alone (0 vs. 15%, two-bottle choice), (2) nicotine alone (0.03 mg/kg/infusion, active vs. inactive lever), and (3) concurrent access to both EtOH and nicotine. Using this model, we examined the effects of (1) varenicline, a nicotinic acetylcholine receptor (nAChR) partial agonist with high affinity for the α4β2* subtype; (2) r-bPiDI, a subtype-selective antagonist at α6β2* nAChRs; and (3) (R)-modafinil, an atypical inhibitor of the dopamine transporter (DAT).

RESULTS

In phases 1 and 2, pharmacologically relevant intake of EtOH and nicotine was achieved. In the concurrent access phase (phase 3), EtOH consumption decreased while nicotine intake increased relative to phases 1 and 2. For drug pretreatments, in the EtOH access phase (phase 1), (R)-modafinil (100 mg/kg) decreased EtOH consumption, with no effect on water consumption. In the concurrent access phase, varenicline (3 mg/kg), r-bPiDI (20 mg/kg), and (R)-modafinil (100 mg/kg) decreased nicotine self-administration but did not alter EtOH consumption, water consumption, or inactive lever pressing.

CONCLUSIONS

These results indicate that therapeutics which may be useful for smoking cessation via selective inhibition of α4β2* or α6β2* nAChRs, or DAT inhibition, may not be sufficient to treat EtOH and nicotine co-use.

The nicotinic receptor drug sazetidine-A reduces alcohol consumption in mice without affecting concurrent nicotine consumption

Alcohol and nicotine addiction are frequently co-morbid. The nicotinic acetylcholine receptors (nAChRs) are critical for both alcohol and nicotine addiction mechanisms, since nAChR drugs that reduce nicotine consumption have been shown to also reduce alcohol consumption. Sazetidine-A, a pre-clinical nAChR drug with agonist and desensitizing effects at α4β2 and α7 nAChRs, has been reported to reduce alcohol consumption and nicotine self-administration in rats when administered at high doses. However, this effect has not been replicated in mice. In this study, we examined the effect of sazetidine-A on alcohol and nicotine consumption in male and female mice utilizing voluntary oral consumption procedures previously developed in our lab. We found that sazetidine-A (1 mg/kg, i.p) reduced overnight alcohol consumption, but did not affect nicotine consumption when presented either alone or concurrently with alcohol. Sazetidine-A did not reduce water or saccharin consumption at any dose tested. In a chronic co-consumption experiment in which either alcohol or nicotine was re-introduced after one week of forced abstinence, sazetidine-A attenuated post-abstinence consumption of alcohol but not nicotine. Sazetidine-A also significantly reduced alcohol consumption in an acute, binge drinking-in-the-dark procedure. Finally, we tested the effect of sazetidine-A on alcohol withdrawal, and found that sazetidine-A significantly reduced handling-induced convulsions during alcohol withdrawal. Collectively, these data suggest a novel role for the nAChR targets of sazetidine-A in specifically mediating alcohol consumption, separate from the involvement of nAChRs in mediating nicotine consumption. Delineation of this pathway may provide insight into novel therapies for the treatment of alcohol use disorders.

The Effects of Long-Term Varenicline Administration on Ethanol and Sucrose Seeking and Self-Administration in Male P Rats

BACKGROUND

Varenicline, a partial agonist at α4β2 and full agonist at α7 nicotinic cholinergic receptors, is FDA-approved for treatment of smoking cessation and has been found to reduce alcohol craving in clinical populations. In rodents, varenicline decreases free-choice ethanol (EtOH) intake with somewhat mixed findings in operant paradigms that utilize a combined appetitive/consummatory response.

METHODS

The present experiment utilized an operant paradigm that procedurally separates appetitive from consummatory responding and a "reward-blocking" approach (i.e., rats were able to consume EtOH during treatment) to better understand the efficacy of varenicline as a treatment for EtOH self-administration and subsequent EtOH seeking. Separate groups of EtOH- and sucrose-reinforced alcohol-preferring, male P rats experienced alternating cycles of vehicle (2-week cycles) and varenicline (0.3, 1.0, and 2.0 mg/kg self-administered in a gelatin preparation) treatment (3-week cycles) prior to daily sessions where a single lever press resulted in 20 minutes of reinforcer access. At the end of each cycle, a single extinction session assessed the seeking response in the absence of drug pretreatment.

RESULTS

Varenicline dose dependently decreased EtOH intake. Sucrose intake was largely unaffected, with no overall treatment effects and only sporadic days where the medium and high dose differed from vehicle. Neither sucrose nor EtOH seeking was significantly decreased by varenicline, and there were no treatment effects on either lick or lever-press latency. Overall effect sizes were much greater for both drinking and seeking in the EtOH group as compared to the sucrose group.

CONCLUSIONS

Varenicline effectively attenuates EtOH self-administration during treatment, but the experience with EtOH consumption while varenicline is "on board" is not sufficient to alter subsequent EtOH seeking. The overall pattern of findings indicates that varenicline blocks the rewarding properties of EtOH while not substituting for EtOH, that the nonspecific effects on an alternate reinforcer are negligible, and that blood levels of varenicline need to be maintained in order for treatment to remain effective.

Advancing Pharmacotherapy Development from Preclinical Animal Studies

Animal models provide rapid, inexpensive assessments of an investigational drug's therapeutic potential. Ideally, they support the plausibility of therapeutic efficacy and provide a rationale for further investigation. Here, I discuss how the absence of clear effective-ineffective categories for alcohol use disorder (AUD) medications and biases in the clinical and preclinical literature affect the development of predictive preclinical alcohol dependence (AD) models. Invoking the analogical argument concept from the philosophy of science field, I discuss how models of excessive alcohol drinking support the plausibility of clinical pharmacotherapy effects. Even though these models are not likely be completely discriminative, they are sensitive to clinically effective medications and have revealed dozens of novel medication targets. In that context, I discuss recent preclinical work on GLP-1 receptor agonists, phosphodiesterase inhibitors, glucocorticoid receptor antagonists, nociception agonists and antagonists, and CRF1 antagonists. Clinically approved medications are available for each of these drug classes. I conclude by advocating a translational approach in which drugs are evaluated highly congruent preclinical models and human laboratory studies. Once translation is established, I suggest the burden is to develop hypothesis-based therapeutic interventions maximizing the impact of the confirmed pharmacotherapeutic effects in the context of additional variables falling outside the model.

Nicotine as a discriminative stimulus for ethanol use

Abused drugs reinforce behavior; i.e., they increase the probability of the behavior preceding their administration. Abused drugs can also act as discriminative stimuli; i.e., they can set the occasion for responding reinforced by another event. Thus, one abused drug could come to set the occasion for the use of another and this functional relationship may play a role in polysubstance abuse, where common patterns of use could result in this relationship. Here we establish nicotine (0.4mg/kg, ip 5-min pre-session) as a discriminative stimulus for behavior reinforced by ethanol (0.1ml 8% w/v po, versus food) and determine the ability of nicotine (0.02-0.4mg/kg), varenicline (0.1-3.0mg/kg), and ethanol (250 and 500mg/kg) to control responding for ethanol. We compare these results to those from rats where nicotine signaled food was available (and ethanol was not). Nicotine came to function as a discriminative stimulus. Nicotine and varenicline produced dose-dependent increases in responding on the nicotine-appropriate lever while ethanol produced responding on the vehicle-appropriate lever. Whether this responding occurred on the lever that produced ethanol or food access depended on the training condition. These results demonstrate that a drug can come to set the occasion for use of another and suggest that this behavioral mechanism could play an important role in the maintenance of and recovery from polysubstance abuse, depending on the pattern of use.

The Corticotropin Releasing Factor Receptor 1 in Alcohol Use Disorder: Still a Valid Drug Target?

Corticotropin releasing factor (CRF) is a neuropeptide that plays a key role in behavioral and physiological responses to stress. A large body of animal literature implicates CRF acting at type 1 CRF receptors (CRFR1) in consumption by alcohol-dependent subjects, stress-induced reinstatement of alcohol seeking, and possibly binge alcohol consumption. These studies have encouraged recent pilot studies of CRFR1 antagonists in humans with alcohol use disorder (AUD). It was a great disappointment to many in the field that these studies failed to show an effect of these compounds on stress-induced alcohol craving. Here, we examine these studies to explore potential limitations and discuss preclinical and human literature to ask whether CRFR1 is still a valid drug target to pursue for the treatment of AUD.

Varenicline Reduces Alcohol Intake During Repeated Cycles of Alcohol Reaccess Following Deprivation in Alcohol-Preferring (P) Rats

BACKGROUND

Most alcoholics experience periods of voluntary alcohol abstinence or imposed alcohol deprivation followed by a return to alcohol drinking. This study examined whether varenicline (VAR) reduces alcohol intake during a return to drinking after periods of alcohol deprivation in rats selectively bred for high alcohol drinking (the alcohol preferring or "P" rats).

METHODS

Alcohol-experienced P rats were given 24-hour access to food and water and scheduled access to alcohol (15% and 30% v/v) for 2 h/d. After 4 weeks, rats were deprived of alcohol for 2 weeks, followed by reaccess to alcohol for 2 weeks, and this pattern was repeated for a total of 3 cycles. Rats were fed either vehicle (VEH) or VAR, in doses of 0.5, 1.0, or 2.0 mg/kg BW, at 1 hour prior to onset of the daily alcohol reaccess period for the first 5 days of each of the 3 alcohol reaccess cycles.

RESULTS

Low-dose VAR (0.5 mg/kg BW) reduced alcohol intake during the 5 days of drug treatment in alcohol reaccess cycles 1 and 2. Higher doses of VAR (1.0 mg/kg BW and 2.0 mg/kg BW) reduced alcohol intake during the 5 days of treatment in all 3 alcohol reaccess cycles. The decrease in alcohol intake disappeared with termination of VAR treatment in all alcohol reaccess cycles.

CONCLUSIONS

The results demonstrate that VAR decreases alcohol intake during multiple cycles of alcohol reaccess following alcohol deprivation in rats and suggests that it may prevent a return to heavy alcohol drinking during a lapse from alcohol abstinence in humans with alcohol use disorder.

Varenicline Reduces Context-Induced Relapse to Alcohol-Seeking through Actions in the Nucleus Accumbens

Varenicline, a pharmacotherapy for tobacco addiction, reduces alcohol consumption in humans and rodents. The therapeutic potential of varenicline would escalate if it also diminished conditioned responses elicited by alcohol-predictive cues, which can precipitate relapse in abstinent individuals. We investigated this application, along with the underlying neural substrates, using a robust preclinical assay in which relapse to alcohol-seeking was triggered by re-exposure to an alcohol-associated environmental context. Male, Long-Evans rats received Pavlovian conditioning sessions in which one auditory conditioned stimulus (CS+) was paired with 15% ethanol and a second conditioned stimulus (CS-) was not. Ethanol was delivered into a port for oral consumption and port entries triggered by each CS were recorded. Extinction was then conducted in a different context where the CS+ and CS- were presented without ethanol. To stimulate relapse, both cues were subsequently presented without ethanol in the prior conditioning context. Systemic varenicline (0, 0.5 or 2.5 mg/kg; intraperitoneal) blocked context-induced relapse to alcohol-seeking without affecting the ability to make a port entry. It also reduced context-induced relapse to sucrose-seeking, but only at the 2.5 mg/kg dose. Neuropharmacological studies showed that context-induced relapse to alcohol-seeking was attenuated by bilateral microinfusion of varenicline (0.3 μl/side) into the nucleus accumbens (NAc; 0 or 3.5 μg), but not the ventral tegmental area (0, 2 or 4 μg). These data show for the first time that varenicline reduces relapse triggered by contexts that predict alcohol, and suggest that nicotinic acetylcholine receptors in the NAc are critical for this effect.

Alcohol and nicotine interactions: pre-clinical models of dependence

While the co-morbidity of alcohol (ethanol) and tobacco (nicotine) dependence is well described, the processes that underpin this strong connection are still under debate. With the increasing popularity of electronic cigarettes (e-cigarettes), it is now becoming more important to look to the neurobiological mechanisms involving alcohol and nicotine interactions to effectively treat a new generation of co-dependent individuals. Researchers have already recognized that the neuropathology produced by the combination of nicotine and ethanol is likely to produce an addictive nature very different to that of either one alone, and are employing a mixture of pre-clinical techniques to establish and investigate every stage in the development of both nicotine and ethanol-seeking behaviors. While it is agreed that multiple pathways orchestrate the complex reward profile of alcohol and nicotine co-addiction, several lines of evidence suggest the convergent site of action is within the mesolimbic dopaminergic system, at neuronal nicotinic acetylcholine receptors (nAChRs). A whole host of strategies are currently being employed to discover and unravel previously unknown or ill understood neurobiological processes in the brain, contributing greatly toward the development of novel pharmacotherapies with the aim of improving patient outcomes. This review intends to shed some light on the most influential and most recent pre-clinical work that is leading the charge in modeling this complicated relationship.

Interactions between nicotine and drugs of abuse: a review of preclinical findings

Polysubstance abuse is common among substance-use disorder patients, and nicotine is one of the most commonly co-used substances. Epidemiological and clinical laboratory studies suggest that nicotine, when combined with other drugs of abuse, increases intake of one or both substances. This review focuses on the preclinical literature regarding nicotine's interaction with alcohol, stimulants (i.e., cocaine, amphetamines), opioids (i.e., morphine, heroin), and Δ⁹-tetrahydrocannabinol (THC). The current understanding of how these various classes of abused drugs may interact with nicotine on behavioral, physiological, and pharmacological indices that may be important in maintaining co-use of one or both substances in human populations are highlighted. Suggestions as to future areas of research and gaps in knowledge are offered.

Mechanisms and genetic factors underlying co-use of nicotine and alcohol or other drugs of abuse

Concurrent use of tobacco and alcohol or psychostimulants represents a major public health concern, with use of one substance influencing consumption of the other. Co-abuse of these drugs leads to substantial negative health outcomes, reduced cessation, and high economic costs, but the underlying mechanisms are poorly understood. Epidemiological data suggest that tobacco use during adolescence plays a particularly significant role. Adolescence is a sensitive period of development marked by major neurobiological maturation of brain regions critical for reward processing, learning and memory, and executive function. Nicotine exposure during this time produces a unique and long-lasting vulnerability to subsequent substance use, likely via actions at cholinergic, dopaminergic, and serotonergic systems. In this review, we discuss recent clinical and preclinical data examining the genetic factors and mechanisms underlying co-use of nicotine and alcohol or cocaine and amphetamines. We evaluate the critical role of nicotinic acetylcholine receptors throughout, and emphasize the dearth of preclinical studies assessing concurrent drug exposure. We stress important age and sex differences in drug responses, and highlight a brief, low-dose nicotine exposure paradigm that may better model early use of tobacco products. The escalating use of e-cigarettes among youth necessitates a closer look at the consequences of early adolescent nicotine exposure on subsequent alcohol and drug abuse.

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

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

The 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.