Effects of varenicline on the reinforcing and discriminative stimulus effects of cocaine in rhesus monkeys

E-cigarettes may serve as a gateway to conventional cigarettes and other addictive drugs

Electronic cigarettes (e-cigarettes) are devices that allow the user to inhale nicotine in a vapor, and are primarily marketed as a means of quitting smoking and a less harmful replacement for traditional cigarette smoking. However, further research is needed to determine if vaping nicotine via e-cigarettes can be effective. Conversely, nicotine has been considered a gateway drug to alcohol and other addictive drugs and e-cigarettes containing nicotine may have the same effects. Previous reports have shown that e-cigarette use may open the gate for the use of other drugs including conventional cigarettes, cannabis, opioids, etc. The increasing prevalence of e-cigarettes, particularly among youth and adolescents in the last decade have led to an increase in the dual use of e-cigarettes with alcohol, cannabis, and other illicit drug use like heroin and 3-4-methylenedioxymethamphetamine (MDMA). The advent of e-cigarettes as a device to self-administer addictive agents such as cocaine and synthetic cathinones may bring about additional adverse health effects associated with their concurrent use. This review aims to briefly describe e-cigarettes and their different generations, and their co-use with other addictive drugs as well as the use of the device as a tool to self-administer addictive drugs, such as cocaine, etc.

Varenicline for the Treatment of Cocaine Dependence

OBJECTIVES

Varenicline is a partial agonist at the α2β4 and α6β2 nAChR receptors and a full agonist at α7 receptors. Both α7 and α6β2 receptors are implicated in the neural reward circuitry activated by cocaine use. A preliminary clinical trial suggested that varenicline treatment reduced cocaine use. This trial was intended to replicate and extend the findings of the previous trial.

METHODS

This was a 12-week, double-blind, placebo-controlled clinical trial involving 156 subjects with DSM IV cocaine dependence. Subjects received up to 2 mg of varenicline or identical placebo daily along with weekly relapse prevention psychotherapy. The primary outcome measure was cocaine use measured by thrice-weekly urine drug screens. Additional outcome measures included end of study cocaine abstinence, cocaine craving, cocaine withdrawal symptom severity, cigarette use, and global improvement measure by the Clinical Global Improvement Scale.

RESULTS

End of study cocaine abstinence, measured by urine drug screens during the last 3 weeks of the trial, was not different between groups (8% in the varenicline treated subjects and versus 9% in placebo-treated subjects). Generalized estimating equations analysis of urine drug screen results showed no significant difference between groups in cocaine abstinence over the 12 weeks of the trial. There were no significant differences between the 2 groups in cocaine craving or cocaine withdrawal symptom severity. Varenicline was well-tolerated. There were no medication-associated serious adverse events.

CONCLUSIONS

Varenicline plus cognitive-behavioral therapy does not seem to be an efficacious treatment for cocaine dependence.

Effects of chronic treatment with bupropion on self-administration of nicotine + cocaine mixtures in nonhuman primates

Chronic health problems associated with long-term nicotine use are the leading cause of preventable death in the United States. The use of tobacco products is 3-4 times greater among individuals with cocaine use disorder than that observed in the general population. This may reflect the propensity of nicotine to augment the reinforcing effects of cocaine. However, the mechanism of action of nicotine differs from that of cocaine, which presents a significant challenge for the development of pharmacotherapeutic interventions for the management of nicotine + cocaine polydrug abuse. Bupropion, an FDA-approved smoking cessation aid, has pharmacological actions at both monoamine transporters and nicotinic receptors, suggesting that it may be effective at decreasing nicotine + cocaine coabuse. Here, rhesus monkeys (n = 4) responded for food pellets and, separately, intravenous injections of nicotine, cocaine, or nicotine + cocaine mixtures under a second-order FR2(VR16:S) schedule of reinforcement during 7- to 10-day continuous treatment with saline or bupropion (1.0 and 1.8 mg/kg/hr). Results show that bupropion treatment dose-dependently decreased self-administration of nicotine combined with a low dose of cocaine (0.0032 mg/kg/inj); however, when the dose of cocaine in the mixture was higher (i.e., 0.01 mg/kg/inj), bupropion attenuated self-administration in only a subset of subjects. The effective dosage of bupropion increased responding for cocaine alone, nicotine alone, and for saline injections and significantly increased measures of daily activity. The apparent stimulant-like effects of bupropion at the dosage required to decrease cocaine + nicotine self-administration does not support its clinical use for the management of nicotine + cocaine polydrug abuse. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

Cocaine potently blocks neuronal α3β4 nicotinic acetylcholine receptors in SH-SY5Y cells

Cocaine is one of the most abused illicit drugs worldwide. It is well known that the dopamine (DA) transporter is its major target; but cocaine also acts on other targets including nicotinic acetylcholine receptors (nAChRs). In this study, we investigated the effects of cocaine on a special subtype of neuronal nAChR, α3β4-nAChR expressed in native SH-SY5Y cells. α3β4-nAChR-mediated currents were recorded using whole-cell recordings. Drugs were applied using a computer-controlled U-tube drug perfusion system. We showed that bath application of nicotine induced inward currents in a concentration-dependent manner with an EC50 value of 20 µM. Pre-treatment with cocaine concentration-dependently inhibited nicotine-induced current with an IC50 of 1.5 μM. Kinetic analysis showed that cocaine accelerated α3β4-nAChR desensitization, which caused a reduction of the amplitude of nicotine-induced currents. Co-application of nicotine and cocaine (1.5 μM) depressed the maximum response on the nicotine concentration-response curve without changing the EC50 value, suggesting a non-competitive mechanism. The cocaine-induced inhibition of nicotine response exhibited both voltage- and use-dependence, suggesting an open-channel blocking mechanism. Furthermore, intracellular application of GDP-βS (via recording electrode) did not affect cocaine-induced inhibition, suggesting that cocaine did not alter receptor internalization. Moreover, intracellular application of cocaine (30 µM) failed to alter the nicotine response. Finally, cocaine (1.5 μM) was unable to inhibit the nicotine-induced inward current in heterologous expressed α6/α3β2β3-nAChRs and α4β2-nAChRs expressed in human SH-EP1 cells. Collectively, our results suggest that cocaine is a potent blocker for native α3β4-nAChRs expressed in SH-SY5Y cells.

Effects of stimulation of mu opioid and nociceptin/orphanin FQ peptide (NOP) receptors on alcohol drinking in rhesus monkeys

Alcohol use disorder (AUD) persists as a devastating public health problem; widely effective pharmacological treatments are needed. Evidence from rodent models suggests that stimulating brain receptors for the neuropeptide nociceptin/orphanin FQ (NOP) can decrease ethanol drinking. We characterized the effects of the mu opioid peptide (MOP) receptor agonist buprenorphine and the buprenorphine analog (2S)-2-[(5R,6R,7R,14S)-N-cyclopropylmethyl-4,5-epoxy-6,14-ethano-3-hydroxy-6 methoxymorphinan-7-yl]-3,3-dimethylpentan-2-ol (BU08028), which stimulates MOP and NOP receptors, in a translational nonhuman primate model of AUD. Rhesus monkeys drank a 4% ethanol solution 6 h per day, 5 days per week via an operant behavioral panel in their home cages. To assess behavioral selectivity, monkeys responded via a photo-optic switch to earn food pellets. After characterizing the acute effects of BU08028 (0.001-0.01 mg/kg, i.m.) and buprenorphine (0.003-0.056 mg/kg, i.m.), the drugs were administered chronically using a model of pharmacotherapy assessment that incorporates clinical aspects of AUD and treatment. Acutely, both drugs decreased ethanol drinking at doses that did not affect food-maintained responding. During chronic treatment, effects of BU08028 and buprenorphine were maintained for several weeks without development of tolerance or emergence of adverse effects. BU08028 was ~0.5 and 1.0 log units more potent in acute and chronic studies, respectively. The selective NOP receptor agonist SCH 221510 also selectively decreased ethanol intakes when given acutely (0.03-1.0 mg/kg, i.m.), whereas the MOP antagonist naltrexone (1.7-5.6 mg/kg, i.m.) decreased both ethanol intake and food pellets delivered. These data demonstrate that bifunctional MOP/NOP agonists, which may have therapeutic advantages to MOP-selective drugs, can decrease alcohol drinking in nonhuman primates.

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

Methamphetamine is a potent psychostimulant with high abuse rates. Currently, there is no Food and Drug Administration-approved pharmacotherapy for methamphetamine addiction. Ideally, a pharmacotherapy should selectively decrease methamphetamine self-administration without affecting responding for other reinforcers. One way to test this is with the use of a multiple schedule of reinforcement, in which drug and food are available in alternating components within a session. The present study evaluated GZ-793A, a vesicular monoamine transporter-2 inhibitor, and varenicline, a partial agonist at α4β2 and full agonist at α7 nicotinic acetylcholine receptors, for their ability to decrease methamphetamine and food self-administration using a multiple schedule of reinforcement. Male Sprague-Dawley rats self-administered methamphetamine (0.03 mg/kg/intravenous infusion) and food pellets under a multiple schedule of reinforcement. GZ-793A or varenicline was administered before multiple schedule sessions. GZ-793A (5 and 20 mg/kg) significantly decreased methamphetamine intake compared with saline and did not alter food-maintained responding. In contrast, varenicline decreased methamphetamine intake less specifically across time. The results suggest that vesicular monoamine transporter-2 inhibition may be a viable pharmacological target for the treatment of methamphetamine-use disorders.

Discriminative Stimulus Properties of S(-)-Nicotine: "A Drug for All Seasons"

S(-)-Nicotine is the major pharmacologically active substance in tobacco and can function as an effective discriminative stimulus in both experimental animals and humans. In this model, subjects must detect and communicate the nicotine drug state versus the non-drug state. This review describes the usefulness of the procedure to study nicotine, presents a general overview of the model, and provides some relevant methodological details for the establishment of this drug as a stimulus. Once established, the (-)-nicotine stimulus can be characterized for dose response and time course effects. Moreover, tests can be conducted to determine the similarity of effects produced by test drugs to those produced by the training dose of nicotine. Such tests have shown that the stimulus effects of nicotine are stereoselective [S(-)-nicotine >R(+)-nicotine] and that other "natural" tobacco alkaloids and (-)-nicotine metabolites can produce (-)-nicotine-like effects, but these drugs are much less potent than (-)-nicotine. Stimulus antagonism tests with mecamylamine and DHβE (dihydro-β-erythroidine) indicate that the (-)-nicotine stimulus is mediated via α4β2 nicotinic acetylcholine receptors (nAChRs) in brain; dopamine systems also are likely involved. Individuals who try to cease their use of nicotine-based products are often unsuccessful. Bupropion (Zyban®) and varenicline (Chantix®) may be somewhat effective as anti-smoking medications because they probably produce stimulus effects that serve as suitable substitutes for (-)-nicotine in the individual who is motivated to quit smoking. Finally, it is proposed that future drug discrimination studies should apply the model to the issue of maintenance of abstinence from (-)-nicotine-based products.

Cocaine Directly Inhibits α6-Containing Nicotinic Acetylcholine Receptors in Human SH-EP1 Cells and Mouse VTA DA Neurons

Alpha6-containing nicotinic acetylcholine receptors are primarily found in neurons of the midbrain dopaminergic (DA) system, suggesting these receptors are potentially involved in drug reward and dependence. Here, we report a novel effect that cocaine directly inhibits α6N/α3Cβ2β3-nAChR (α6*-nAChRs) function. Human α6*-nAChRs were heterologously expressed within cells of the SH-EP1 cell line for functional characterization. Mechanically dissociated DA neurons from mouse ventral tegmental area (VTA) were used as a model of presynaptic α6*-nAChR activation since this method preserves terminal boutons. Patch-clamp recordings in whole-cell configuration were used to measure α6*-nAChR function as well as evaluate the effects of cocaine. In SH-EP1 cells containing heterologously expressed human α6*-nAChRs, cocaine inhibits nicotine-induced inward currents in a concentration-dependent manner with an IC₅₀ value of 30 μM. Interestingly, in the presence of 30 μM cocaine, the maximal current response of the nicotine concentration-response curve is reduced without changing nicotine's EC₅₀ value, suggesting a noncompetitive mechanism. Furthermore, analysis of whole-cell current kinetics demonstrated that cocaine slows nAChR channel activation but accelerates whole-cell current decay time. Our findings demonstrate that cocaine-induced inhibition occurs solely with bath application, but not during intracellular administration, and this inhibition is not use-dependent. Additionally, in Xenopus oocytes, cocaine inhibits both α6N/α3Cβ2β3-nAChRs and α6M211L/α3ICβ2β3-nCAhRs similarly, suggesting that cocaine may not act on the α3 transmembrane domain of chimeric α6N/α3Cβ2β3-nAChR. In mechanically isolated VTA DA neurons, cocaine abolishes α6*-nAChR-mediated enhancement of spontaneous inhibitory postsynaptic currents (sIPSCs). Collectively, these studies provide the first evidence that cocaine directly inhibits the function of both heterologously and naturally expressed α6*-nAChRs. These findings suggest that α6*-nAChRs may provide a novel pharmacological target mediating the effects of cocaine and may underlie a novel mechanism of cocaine reward and dependence.

Evaluation of the "Pipeline" for Development of Medications for Cocaine Use Disorder: A Review of Translational Preclinical, Human Laboratory, and Clinical Trial Research

Cocaine use disorder is a persistent public health problem for which no widely effective medications exist. Self-administration procedures, which have shown good predictive validity in estimating the abuse potential of drugs, have been used in rodent, nonhuman primate, and human laboratory studies to screen putative medications. This review assessed the effectiveness of the medications development process regarding pharmacotherapies for cocaine use disorder. The primary objective was to determine whether data from animal and human laboratory self-administration studies predicted the results of clinical trials. In addition, the concordance between laboratory studies in animals and humans was assessed. More than 100 blinded, randomized, fully placebo-controlled studies of putative medications for cocaine use disorder were identified. Of the 64 drugs tested in these trials, only 10 had been examined in both human and well-controlled animal laboratory studies. Within all three stages, few studies had been conducted for each drug and when multiple studies had been conducted conclusions were sometimes contradictory. Overall, however, there was good concordance between animal and human laboratory results when the former assessed chronic drug treatment. Although only seven of the ten reviewed drugs showed fully concordant results across all three types of studies reviewed, the analysis revealed several subject-related, procedural, and environmental factors that differ between the laboratory and clinical trial settings that help explain the disagreement for other drugs. The review closes with several recommendations to enhance translation and communication across stages of the medications development process that will ultimately speed the progress toward effective pharmacotherapeutic strategies for cocaine use disorder.

Methamphetamine-like discriminative stimulus effects of bupropion and its two hydroxy metabolites in male rhesus monkeys

The dopamine transporter (DAT) inhibitor and nicotinic acetylcholine (nACh) receptor antagonist bupropion is being investigated as a candidate 'agonist' medication for methamphetamine addiction. In addition to its complex pharmacology, bupropion also has two distinct pharmacologically active metabolites. However, the mechanism by which bupropion produces methamphetamine-like 'agonist' effects remains unknown. The aim of the present study was to determine the role of DAT inhibition, nACh receptor antagonism, and the hydroxybupropion metabolites in the methamphetamine-like discriminative stimulus effects of bupropion in rhesus monkeys. In addition, varenicline, a partial agonist at the nACh receptor, and risperidone, a dopamine antagonist, were tested as controls. Monkeys (n=4) were trained to discriminate 0.18 mg/kg intramuscular methamphetamine from saline in a two-key food-reinforced discrimination procedure. The potency and time course of methamphetamine-like discriminative stimulus effects were determined for all compounds. Bupropion, methylphenidate, and 2S,3S-hydroxybupropion produced full, at least 90%, methamphetamine-like effects. 2R,3R-Hydroxybupropion, mecamylamine, and nicotine also produced full methamphetamine-like effects, but drug potency was more variable between monkeys. Varenicline produced partial methamphetamine-like effects, whereas risperidone did not. Overall, these results suggest DAT inhibition as the major mechanism of the methamphetamine-like 'agonist' effects of bupropion, although nACh receptor antagonism appeared, at least partially, to contribute. Furthermore, the contribution of the 2S,3S-hydroxybupropion metabolite could not be completely ruled out.

The discriminative stimulus effects of i.v. nicotine in rhesus monkeys: Pharmacokinetics and apparent pA2 analysis with dihydro-β-erythroidine

Quantitative analysis of antagonism is infrequently used to identify nAChRs mediating behavioral effects. Here, nicotine (0.032 mg/kg i.v.) was established as a discriminative stimulus in rhesus monkeys responding under a fixed ratio 5 schedule; pharmacokinetics and underlying nAChR mechanism(s) were examined. When measured up to 4 h in venous blood, the training dose resulted in the following mean pharmacokinetic parameters: nicotine C_max = 71.7 ng/ml, t_1/2 = 116 min, and clearance = 6.25 ml/min/kg; cotinine C_max = 191 ng/ml; and 3OH-cotinine C_max = 63 ng/ml. The ED₅₀ value of nicotine to produce discriminative stimulus effects was 0.013 mg/kg. Epibatidine and varenicline increased drug-lever responding to 97% and 95%, respectively (ED₅₀ values = 0.00015 and 0.031 mg/kg, respectively), whereas cocaine, midazolam, and morphine produced no more than 28% drug-appropriate responding. Mecamylamine and dihydro-β-erythroidine (DHβE) dose-dependently attenuated the discriminative stimulus effects of the nicotine training dose, whereas methyllycaconitine (MLA) did not. DHβE (0.1 and 0.32) produced rightward shifts of the nicotine and varenicline dose-response functions; Schild plots fitted through individual data resulted in slopes that were not different from unity; the apparent pA₂ calculated for DHβE did not significantly differ in the presence of nicotine (6.58) or varenicline (6.45). Compared to human cigarette smoking, nicotine blood levels after 0.032 mg/kg nicotine i.v. took a similar time to reach maximal concentration, levels at Cmax were similar to smoking 2-3 cigarettes, while average nicotine levels were comparable to smoking 5-6 cigarettes. Apparent pA₂ analysis with DHβE under these conditions is consistent with nicotine and varenicline acting through the same nAChRs to produce discriminative stimulus effects.

Attenuated nicotine-like effects of varenicline but not other nicotinic ACh receptor agonists in monkeys receiving nicotine daily

BACKGROUND AND PURPOSE

Chronic treatment can differentially impact the effects of pharmacologically related drugs that differ in receptor selectivity and efficacy.

EXPERIMENTAL APPROACH

The impact of daily nicotine treatment on the effects of nicotinic ACh receptor (nAChR) agonists was examined in two groups of rhesus monkeys discriminating nicotine (1.78 mg·kg-1 base weight) from saline. One group received additional nicotine treatment post-session (1.78 mg·kg-1 administered five times daily, each dose 2 h apart; i.e. Daily group), and the second group did not (Intermittent group).

KEY RESULTS

Daily repeated nicotine treatment produced a time-related increase in saliva cotinine. There was no significant difference in the ED50 values of the nicotine discriminative stimulus between the Daily and Intermittent group. Mecamylamine antagonized the effects of nicotine, whereas dihydro-β-erythroidine did not. Midazolam produced 0% nicotine-lever responding. The nAChR agonists epibatidine, RTI-36, cytisine and varenicline produced >96% nicotine-lever responding in the Intermittent group. The respective maximum effects in the Daily group were 100, 72, 59 and 28%, which shows that the ability of varenicline to produce nicotine-like responding was selectively decreased in the Daily as compared with the Intermittent group. When combined with nicotine, both varenicline and cytisine increased the potency of nicotine to produce discriminative stimulus effects.

CONCLUSION AND IMPLICATIONS

Nicotine treatment has a greater impact on the sensitivity to the effects of varenicline as compared with some other nAChR agonists. Collectively, these results strongly suggest that varenicline differs from nicotine in its selectivity for multiple nAChR subtypes.

Behavioral sensitization of the reinforcing value of food: What food and drugs have in common

Sensitization is a basic property of the nervous system whereby repeated exposure to a stimulus results in an increase in responding to that stimulus. This increase in responding contributes to difficulty with treatment of drug abuse, as stimuli associated with substance use become signals or triggers for drug craving and relapse. Our work over the past decade has applied the theoretical framework of incentive sensitization to overeating. We have shown, in several studies, that lean adults do not commonly demonstrate behavioral sensitization after repeated exposure to snack food, but a subset of obese adults reliably does. This review will discuss this change in behavioral response to repeated consumption of snack food in obese individuals and apply the theoretical framework of incentive sensitization to drugs of abuse to high fat/high sugar snack foods. We will also show data that suggest that behavioral sensitization to repeated administration of snack food is predictive of weight gain, which may enhance its utility as a diagnostic tool for identifying at-risk individuals for obesity. Finally, we will discuss the future directions of this line of research, including studying the phenomenon in children and adolescents and determining if similar principles can be used to increase motivation to eat healthier food. A combination of reductions in unhealthy food intake and increases and healthy food intake is necessary to reduce obesity rates and improve health.

Development of a translational model to screen medications for cocaine use disorder II: Choice between intravenous cocaine and money in humans

BACKGROUND

A medication for treating cocaine use disorder has yet to be approved. Laboratory-based evaluation of candidate medications in animals and humans is a valuable means to demonstrate safety, tolerability and initial efficacy of potential medications. However, animal-to-human translation has been hampered by a lack of coordination. Therefore, we designed homologous cocaine self-administration studies in rhesus monkeys (see companion article) and human subjects in an attempt to develop linked, functionally equivalent procedures for research on candidate medications for cocaine use disorder.

METHODS

Eight (N=8) subjects with cocaine use disorder completed 12 experimental sessions in which they responded to receive money ($0.01, $1.00 and $3.00) or intravenous cocaine (0, 3, 10 and 30mg/70kg) under independent, concurrent progressive-ratio schedules. Prior to the completion of 9 choice trials, subjects sampled the cocaine dose available during that session and were informed of the monetary alternative value.

RESULTS

The allocation of behavior varied systematically as a function of cocaine dose and money value. Moreover, a similar pattern of cocaine choice was demonstrated in rhesus monkeys and humans across different cocaine doses and magnitudes of the species-specific alternative reinforcers. The subjective and cardiovascular responses to IV cocaine were an orderly function of dose, although heart rate and blood pressure remained within safe limits.

CONCLUSIONS

These coordinated studies successfully established drug versus non-drug choice procedures in humans and rhesus monkeys that yielded similar cocaine choice behavior across species. This translational research platform will be used in future research to enhance the efficiency of developing interventions to reduce cocaine use.

Reinforcing effectiveness of nicotine in nonhuman primates: effects of nicotine dose and history of nicotine self-administration

RATIONALE

Despite the high prevalence of nicotine use in humans, robust nicotine self-administration has been difficult to demonstrate in laboratory animals.

OBJECTIVES

A parametric analysis of nicotine self-administration was conducted to study its reinforcing effects in nonhuman primates.

METHODS

Adult rhesus macaques (N = 6) self-administered intravenous (IV) nicotine (0.001-0.1 mg/kg) under a fixed-ratio (FR)1 schedule of reinforcement during daily 90-min sessions. Next, the demand function relating drug intake and response cost was determined by increasing the FR across sessions during the availability of each of several unit doses of nicotine (0.0032-0.032 mg/kg/inj). The reinforcing effects of 0.01 mg/kg/inj cocaine and 1 g banana-flavored food pellets were also determined under similar testing conditions. Finally, the nicotine demand function was re-determined after approximately 8 months of daily IV nicotine self-administration.

RESULTS

IV nicotine self-administration followed an inverted U-shaped pattern, with the peak number of injections maintained by 0.0032 mg/kg/inj. Self-administration of each reinforcer (food pellets, IV cocaine, and IV nicotine) decreased as FR size increased. Application of the exponential model of demand showed that demand elasticity for nicotine was (1) dose-dependent and lowest for 0.0032 mg/kg/inj; (2) for 0.0032 mg/kg/inj, similar to that of food pellets and significantly higher than cocaine; and (3) decreased after 8 months of daily nicotine self-administration.

CONCLUSIONS

These data show that, though high levels of nicotine self-administration can be achieved under simple FR schedules in nonhuman primates, its reinforcing effectiveness is dose-related but limited and may increase over time.

A threshold model for opposing actions of acetylcholine on reward behavior: Molecular mechanisms and implications for treatment of substance abuse disorders

The cholinergic system plays important roles in both learning and addiction. Medications that modify cholinergic tone can have pronounced effects on behaviors reinforced by natural and drug reinforcers. Importantly, enhancing the action of acetylcholine (ACh) in the nucleus accumbens and ventral tegmental area (VTA) dopamine system can either augment or diminish these behaviors. A threshold model is presented that can explain these seemingly contradictory results. Relatively low levels of ACh rise above a lower threshold, facilitating behaviors supported by drugs or natural reinforcers. Further increases in cholinergic tone that rise above a second upper threshold oppose the same behaviors. Accordingly, cholinesterase inhibitors, or agonists for nicotinic or muscarinic receptors, each have the potential to produce biphasic effects on reward behaviors. Pretreatment with either nicotinic or muscarinic antagonists can block drug- or food- reinforced behavior by maintaining cholinergic tone below its lower threshold. Potential threshold mediators include desensitization of nicotinic receptors and biphasic effects of ACh on the firing of medium spiny neurons. Nicotinic receptors with high- and low- affinity appear to play greater roles in reward enhancement and inhibition, respectively. Cholinergic inhibition of natural and drug rewards may serve as mediators of previously described opponent processes. Future studies should evaluate cholinergic agents across a broader range of doses, and include a variety of reinforced behaviors.

Attenuation of cocaine self-administration by chronic oral phendimetrazine in rhesus monkeys

Chronic treatment with the monoamine releaser d-amphetamine has been consistently shown to decrease cocaine self-administration in laboratory studies and clinical trials. However, the abuse potential of d-amphetamine is an obstacle to widespread clinical use. Approaches are needed that exploit the efficacy of the agonist approach but avoid the abuse potential associated with dopamine releasers. The present study assessed the effectiveness of chronic oral administration of phendimetrazine (PDM), a pro-drug for the monoamine releaser phenmetrazine (PM), to decrease cocaine self-administration in four rhesus monkeys. Each day, monkeys pressed a lever to receive food pellets under a 50-response fixed-ratio (FR) schedule of reinforcement and self-administered cocaine (0.003-0.56 mg/kg per injection, i.v.) under a progressive-ratio (PR) schedule in the evening. After completing a cocaine self-administration dose-response curve, sessions were suspended and PDM was administered (1.0-9.0 mg/kg, p.o., b.i.d.). Cocaine self-administration was assessed using the PR schedule once every 7 days while food-maintained responding was studied daily. When a persistent decrease in self-administration was observed, the cocaine dose-effect curve was re-determined. Daily PDM treatment decreased cocaine self-administration by 30-90% across monkeys for at least 4 weeks. In two monkeys, effects were completely selective for cocaine. Tolerance developed to initial decreases in food-maintained responding in the third monkey and in the fourth subject, fluctuations were observed that were lower in magnitude than effects on cocaine self-administration. Cocaine dose-effect curves were shifted down and/or rightward in three monkeys. These data provide further support for the use of agonist medications for cocaine abuse, and indicate that the promising effects of d-amphetamine extend to a more clinically viable pharmacotherapy.

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.

Effects of continuous nicotine treatment and subsequent termination on cocaine versus food choice in male rhesus monkeys

One complicating factor in cocaine addiction may be concurrent exposure and potential dependence on nicotine. The aim of the present study was to determine the effects of continuous nicotine treatment and subsequent termination on cocaine versus food choice in rhesus monkeys (Macaca mulatta). For comparison, we also determined effects of the nicotinic receptor antagonist mecamylamine on cocaine versus food choice during continuous saline and nicotine treatment. Rhesus monkeys (N = 3) responded under a concurrent schedule of food pellet (1 g) and intravenous cocaine (0-0.1 mg/kg/injection) availability. Saline and ascending nicotine doses (0.1-1.0 mg/kg/hr, intravenous) were continuously infused for 7-day treatment periods and separated by 24-hr saline treatment periods. Acute effects of mecamylamine (0.32-1.8 mg/kg, intramuscular, 15 min pretreatment) were determined during continuous saline and 0.32-mg/kg/hr nicotine treatments. During saline treatment, cocaine maintained a dose-dependent increase in cocaine choice. Nicotine treatment did not alter cocaine versus food choice. In contrast, preference of 0.032 mg/kg/injection cocaine was attenuated 24 hr following termination of 0.32-mg/kg/hr nicotine treatment, despite no somatic abstinence signs being observed. Acute mecamylamine enhanced cocaine choice during saline treatment and mainly suppressed rates of behavior during nicotine treatment. Overall, continuous nicotine exposure, up to 1 mg/kg/hr, does not enhance cocaine choice and does not produce nicotine dependence, as demonstrated by the lack of abstinence signs.

The Reinforcing Effects of Nicotine in Humans and Nonhuman Primates: A Review of Intravenous Self-Administration Evidence and Future Directions for Research

INTRODUCTION

Cigarette smoking is largely driven by the reinforcing properties of nicotine. Intravenous (IV) self-administration procedures are the gold standard for investigating the reinforcing effects of psychoactive drugs. The goal of this review was to examine the results of published investigations of the reinforcing effects of nicotine measured using IV self-administration procedures in humans and nonhuman primates.

RESULTS

The body of literature using nonhuman primate subjects indicates nicotine functions as a positive reinforcer when available for self-administration via IV catheters. However, it can also be difficult to establish IV nicotine self-administration in nonhuman primates and sometimes supplemental strategies have been required (e.g., priming injections or food deprivation) before subjects acquire the behavior. Although the body of literature using human subjects is limited, the evidence indicates nicotine functions as a reinforcer via the IV route of administration in adult cigarette smokers. Rates of nicotine self-injection can be variable across subjects and responding is sometimes inconsistent across sessions in both humans and nonhuman primates.

CONCLUSIONS

The Family Smoking Prevention and Tobacco Control Act, enacted in 2009, gave the Food and Drug Administration regulatory authority over the manufacture, marketing, and distribution of tobacco products. Research examining the threshold reinforcing doses for initiation and maintenance of nicotine self-administration, comparisons of the reinforcing effects of nicotine in adolescent versus adult subjects, investigations of gender differences in the reinforcing effects of nicotine, and studies of the abuse liability of non-nicotine tobacco product constituents and their ability to alter the reinforcing effects of nicotine will inform potential tobacco regulatory actions.

Nicotinic receptor modulation to treat alcohol and drug dependence

Alcohol and drug dependence are serious public health problems worldwide. The prevalence of alcohol and drug dependence in the United States and other parts of the world is significant. Given the limitations in the efficacy of current pharmacotherapies to treat these disorders, research in developing alternative pharmacotherapies continues. Preclinical and clinical evidence thus far has indicated that brain nicotinic acetylcholine receptors (nAChRs) are important pharmacological targets for the development of medications to treat alcohol and drug dependence. The nAChRs are a super family of ligand gated ion channels, and are expressed throughout the brain with twelve neuronal nAChR subunits (α2–α10 and β2–β4) identified. Here, we review preclinical and clinical evidence involving a number of nAChR ligands that target different nAChR subtypes in alcohol and nicotine addiction. The important ligands include cytisine, lobeline, mecamylamine, varenicline, sazetidine A and others that target α4β2^* nAChR subtypes as small molecule modulators of the brain nicotinic cholinergic system are also discussed. Taken together, both preclinical and clinical data exist that support nAChR–based ligands as promising therapeutic agents for the treatment of alcohol and drug dependence.

Nicotine enhances the expression of a sucrose or cocaine conditioned place preference in adult male rats

Nicotine has been shown to enhance the motivational properties of non-nicotine stimuli. This reinforcement-enhancing property of nicotine has the potential to promote the use of other illicit substances as well as maladaptive patterns of food intake. Therefore, the current study aimed to examine whether nicotine enhances preference for contexts paired with cocaine or sucrose utilizing a place conditioning procedure. Separate groups of adult male rats were administered sucrose or cocaine in one of two compartments of a standard CPP chamber on four consecutive days. Preference was then assessed following no injection, a single subcutaneous (s.c.) injection of nicotine, and a s.c. saline injection. The animals preferred the chamber paired with either sucrose or cocaine, as evident from an increased time spent in the paired chamber compared to baseline. Nicotine further increased the time spent in the sucrose- or cocaine-paired chamber, consistent with a reinforcement-enhancement effect. Previous results demonstrate an interaction between nicotine and intake of other drugs or food. The present findings provide an additional mechanism that may underlie these effects and which may have implications for drug dependence and obesity.

Short-term, low-dose varenicline administration enhances information processing speed in methamphetamine-dependent users

Long-term, high-dose methamphetamine (METH) use is associated with decrements in neurocognition and, given the association between impaired neurocognition and poorer treatment outcomes in individuals dependent on alcohol and drugs, it is considered to be a neglected area of critical concern. The objective of this study was to determine whether varenicline, a partial agonist at α4β2- and a full agonist at α7-nicotinic acetylcholine receptors, enhances attention/information processing speed, episodic memory, and working memory in non-treatment seeking METH-dependent participants. Twenty-six participants were randomly assigned to receive oral placebo or oral varenicline (titrated up to 1 mg) over 5 days during three separate inpatient phases, and 17 completed each inpatient phase. Participants were predominately male (71%) and Caucasian (71%). Varenicline significantly improved reaction time on the n-back for visual stimuli (F(1,47)=5.369, p=0.025, η2=0.103), and a trend was observed for improvement in reaction time for auditory stimuli (F(1,47)=3.141, p=0.083, η2=0.063). For those study participants whose reaction time was in the lower half of the distribution at baseline, the effect was even more pronounced for auditory (F(1,22)=5.287, p=0.031, η2=0.194) and visual (F(1,22)=11.981, p=0.002, η2=0.353) stimuli relative to placebo. In contrast, varenicline did not modulate mean or maximum span of working memory or performance on tests of episodic memory or attention (p's>0.05). Given the potential importance of this finding, it should be replicated in a larger sample over a longer treatment period with a higher dose of varenicline (2 mg).

TRIAL REGISTRATION

clinicalTrials.gov Identifier NCT01571167.

Effects of the nicotinic acetylcholine receptor antagonist mecamylamine on the discriminative stimulus effects of cocaine in male rhesus monkeys

Preclinical drug discrimination procedures have been useful in understanding the pharmacological mechanisms of the subjective-like effects of abused drugs. Converging lines of evidence from neurochemical and behavioral studies implicate a potential role of nicotinic acetylcholine (nACh) receptors in the abuse-related effects of cocaine. The aim of the present study was to determine the effects of the nACh receptor antagonist mecamylamine on the discriminative stimulus effects of cocaine in nonhuman primates. The effects of mecamylamine on the cocaine-like discriminative stimulus effects of nicotine were also examined. Male rhesus monkeys (n = 5) were trained to discriminate 0.32 mg/kg, IM cocaine from saline in a 2-key, food-reinforced discrimination procedure. Initially, potency and time course of cocaine-like discriminative stimulus effects were determined for nicotine and mecamylamine alone. Test sessions were then conducted examining the effects of mecamylamine on cocaine or the cocaine-like discriminative stimulus effects of nicotine. Curiously, mecamylamine produced partial cocaine-like discriminative stimulus effects. Mecamylamine did not significantly alter the discriminative stimulus effects of cocaine up to doses that significantly decreased rates of operant responding. Mecamylamine and nicotine combinations were not different than saline. These results confirm previous nonhuman primate studies of partial substitution with nicotine and extend these findings with mecamylamine. Furthermore, these results extend previous results in rats suggesting cocaine may have nACh receptor antagonist properties.

Effects of chronic varenicline treatment on nicotine, cocaine, and concurrent nicotine+cocaine self-administration

Nicotine dependence and cocaine abuse are major public health problems, and most cocaine abusers also smoke cigarettes. An ideal treatment medication would reduce both cigarette smoking and cocaine abuse. Varenicline is a clinically available, partial agonist at α4β2* and α6β2* nicotinic acetylcholine receptors (nAChRs) and a full agonist at α7 nAChRs. Varenicline facilitates smoking cessation in clinical studies and reduced nicotine self-administration, and substituted for the nicotine-discriminative stimulus in preclinical studies. The present study examined the effects of chronic varenicline treatment on self-administration of IV nicotine, IV cocaine, IV nicotine+cocaine combinations, and concurrent food-maintained responding by five cocaine- and nicotine-experienced adult rhesus monkeys (Macaca mulatta). Varenicline (0.004-0.04 mg/kg/h) was administered intravenously every 20 min for 23 h each day for 7-10 consecutive days. Each varenicline treatment was followed by saline-control treatment until food- and drug-maintained responding returned to baseline. During control treatment, nicotine+cocaine combinations maintained significantly higher levels of drug self-administration than nicotine or cocaine alone (P<0.05-0.001). Varenicline dose-dependently reduced responding maintained by nicotine alone (0.0032 mg/kg/inj) (P<0.05), and in combination with cocaine (0.0032 mg/kg/inj) (P<0.05) with no significant effects on food-maintained responding. However, varenicline did not significantly decrease self-administration of a low dose of nicotine (0.001 mg/kg), cocaine alone (0.0032 and 0.01 mg/kg/inj), or 0.01 mg/kg cocaine combined with the same doses of nicotine. We conclude that varenicline selectively attenuates the reinforcing effects of nicotine alone but not cocaine alone, and its effects on nicotine+cocaine combinations are dependent on the dose of cocaine.

Prior exposure to THC increases the addictive effects of nicotine in rats

Although it is more common for drug abuse to progress from tobacco to cannabis, in many cases cannabis use develops before tobacco use. Epidemiological evidence indicates that prior cannabis use increases the likelihood of becoming dependent on tobacco. To determine whether this effect might be due to cannabis exposure per se, in addition to any genetic, social, or environmental factors that might contribute, we extended our series of studies on 'gateway drug' effects in animal models of drug abuse. Rats were exposed to THC, the main psychoactive constituent of cannabis, for 3 days (two intraperitoneal injections/day). Then, starting 1 week later, they were allowed to self-administer nicotine intravenously. THC exposure increased the likelihood of acquiring the nicotine self-administration response from 65% in vehicle-exposed rats to 94% in THC-exposed rats. When the price of nicotine was manipulated by increasing the response requirement, THC-exposed rats maintained higher levels of intake than vehicle-exposed rats, indicating that THC exposure increased the value of nicotine reward. These results contrast sharply with our earlier findings that prior THC exposure did not increase the likelihood of rats acquiring either heroin or cocaine self-administration, nor did it increase the reward value of these drugs. The findings obtained here suggest that a history of cannabis exposure might have lasting effects that increase the risk of becoming addicted to nicotine.

The influence of smoking cigarettes on the high and desire for cocaine among active cocaine users

The primary aim was to evaluate whether nicotine use alters the high or desire for cocaine among active cocaine users who concurrently smoke cigarettes. Participants answered the Fagerstrom Test for Nicotine Dependence (FTND), Nicotine-Stimulant Interaction Questionnaire (NSIQ), and Multiple Drug Use Questionnaire (MDUQ). These questionnaires employ subject recall of participants' drug use habits. The participants that smoked (N=163/188) were primarily African American males who were 45.0±0.5 (mean±S.E.M.) years of age, and used cocaine for 17.9±0.6 years and 19.8±0.6 days out of the last 30. These individuals smoked 14.0±0.8 cigarettes/day (CPD), scored 4.6±0.2 (on a scale of 0-10) on the FTND, and smoked cigarettes for 23.5±0.7 years. Two questions from the MDUQ, which evaluates the interaction between cocaine and nicotine, (-5: reduces effect, 0: no change, +5: increases effect) included "Does nicotine affect the high that you experience from cocaine?" and "Does nicotine affect your desire for cocaine?", and the scores were 1.3±0.2 and 0.8±0.2, respectively. The NSIQ also evaluated interactive effects of nicotine and cocaine, on a scale of 0 to 100 (0: not at all, 100: most ever). Smokers responded most strongly that using cocaine increased both the urge to smoke and cigarette craving. Additional analyses were performed by separating participants into HighCPD vs. LowCPD groups via median split. The HighCPD group smoked 22.7±1.1 CPD while the LowCPD group smoked 6.4±0.3 CPD [F(1,161)=228.4, p<0.0001], and the HighCPD group had a mean FTND score twice that of the LowCPD group. Significant differences emerged between the two groups on multiple items of the NSIQ, but not the MDUQ. The subjective ratings of high and desire for cocaine, and several subjective effects produced by cocaine, were modestly altered by cigarette smoking. Taken together, these data suggest that cigarette smoking may augment the craving and high produced by cocaine.

Effects of varenicline on ethanol- and food-maintained responding in a concurrent access procedure

BACKGROUND

Varenicline has been reported to reduce drinking in smokers and to selectively decrease responding for ethanol (EtOH) versus alternatives in preclinical studies. Such selectivity may reflect potential therapeutic effects and the involvement of nicotinic receptors in EtOH reinforcement. However, these studies have been conducted with EtOH and an alternative available in isolation or in separate groups, and selectivity can depend on the context in which reinforcement occurs. Whether varenicline selectivity is maintained when EtOH and an alternative are concurrently available has not been reported. To examine the effects of varenicline on EtOH self-administration when an alternative is concurrently available, male Lewis rats (n = 5) were trained to respond for EtOH and food under a concurrent FR5 FRX schedule where the fixed ratio (FR) for food was adjusted (FR = 25 or 35 for each subject) to provide similar numbers of EtOH and food deliveries during a 30-minute session.

METHODS

Doses of varenicline (0.56 to 5.6 mg/kg, i.p.) or vehicle were administered 30 minutes before sessions. Effects of varenicline on responding across the session and during each tenth of the session were compared to responding following vehicle treatment.

RESULTS

Lower doses (0.56 to 1.0 mg/kg) of varenicline increased responding for EtOH without affecting responding for food. Higher doses disrupted responding for EtOH and food similarly.

CONCLUSIONS

Previous reports of varenicline selectivity on EtOH-maintained responding do not generalize to other experimental conditions such as a concurrent schedule. The increase in responding for EtOH following lower doses might be due to enhanced EtOH reinforcement, decreased food reinforcement, rate dependency, or greater perseverance on the initial, EtOH response.

Effects of nicotinic acetylcholine receptor agonists on cognition in rhesus monkeys with a chronic cocaine self-administration history

Cocaine use is associated with impaired cognitive function, which may negatively impact treatment outcomes. One pharmacological strategy to improve cognition involves nicotinic acetylcholine receptor (nAChR) stimulation. However, the effects of chronic cocaine exposure on nAChR distribution and function have not been characterized. Thus, one goal of this study was to examine nAChR availability in rhesus monkeys with an extensive cocaine self-administration history (n = 4; ~6 years, mean intake, 1463 mg/kg) compared to age-matched cocaine-naive control monkeys (n = 5). Using [¹¹C]-nicotine and positron emission tomography (PET) imaging, cocaine-experienced monkeys showed significantly higher receptor availability in the hippocampus compared to cocaine-naive monkeys. A second goal was to examine the effects of nAChR agonists on multiple domains of cognitive performance in these same monkeys. For these studies, working memory was assessed using a delayed match-to-sample (DMS) task, associative learning and behavioral flexibility using stimulus discrimination and reversal learning tasks. When administered acutely, the nonselective high-efficacy agonist nicotine, the low-efficacy α4β2* subtype-selective agonist varenicline and the high-efficacy α7 subtype-selective agonist, PNU-282987 significantly improved DMS performance in both cocaine-naive and cocaine-experienced monkeys. Individual doses of nicotine and varenicline that engendered maximum cognitive enhancing effects on working memory did not affect discrimination or reversal learning, while PNU-282987 disrupted reversal learning in the cocaine-naive monkeys. These findings indicate that a cocaine self-administration history influenced nAChR distribution and the effects of nAChR agonists on cognitive performance, including a reduced sensitivity to the disrupting effects on reversal learning. The cognitive enhancing effects of nAChR agonists may be beneficial in combination with behavioral treatments for cocaine addiction. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

Pharmacologic characterization of a nicotine-discriminative stimulus in rhesus monkeys

This study examined mechanisms by which nicotine (1.78 mg/kg base s.c.) produces discriminative stimulus effects in rhesus monkeys. In addition to nicotine, various test compounds were studied including other nicotinic acetylcholine receptor agonists (varenicline and cytisine), antagonists [mecamylamine and the α4β2 receptor-selective antagonist dihydro-β-erythroidine (DHβE)], a nicotinic acetylcholine receptor antagonist/indirect-acting catecholamine agonist (bupropion), and non-nicotinics (cocaine and midazolam). Nicotine, varenicline, and cytisine dose-dependently increased drug-lever responding; the ED(50) values were 0.47, 0.53, and 39 mg/kg, respectively. Bupropion and cocaine produced 100% nicotine-lever responding in a subset of monkeys, whereas mecamylamine, DHβE, and midazolam produced predominantly vehicle-lever responding. The training dose of nicotine resulted in 1128 ng/ml cotinine in saliva. Mecamylamine antagonized the discriminative stimulus effects of nicotine and varenicline, whereas DHβE was much less effective. Nicotine and varenicline had synergistic discriminative stimulus effects. In monkeys responding predominantly on the vehicle lever after a test compound (bupropion, cocaine, and midazolam), that test compound blocked the nicotine-discriminative stimulus, perhaps reflecting a perceptual-masking phenomenon. These results show that nicotine, varenicline, and cytisine produce discriminative stimulus effects through mecamylamine-sensitive receptors (i.e., nicotinic acetylcholine) in primates, whereas the involvement of DHβE-sensitive receptors (i.e., α4β2) is unclear. The current nicotine-discrimination assay did not detect a difference in agonist efficacy between nicotine, varenicline, and cytisine, but did show evidence of involvement of dopamine. The control that nicotine has over choice behavior can be disrupted by non-nicotinic compounds, suggesting that non-nicotinics could be exploited to decrease the control that tobacco has over behavior.