Altering the motivational function of nicotine through conditioning processes

Behavioral mechanisms underlying nicotine reinforcement

Cigarette smoking is the leading cause of preventable deaths worldwide, and nicotine, the primary psychoactive constituent in tobacco, drives sustained use. The behavioral actions of nicotine are complex and extend well beyond the actions of the drug as a primary reinforcer. Stimuli that are consistently paired with nicotine can, through associative learning, take on reinforcing properties as conditioned stimuli. These conditioned stimuli can then impact the rate and probability of behavior and even function as conditioning reinforcers that maintain behavior in the absence of nicotine. Nicotine can also act as a conditioned stimulus (CS), predicting the delivery of other reinforcers, which may allow nicotine to acquire value as a conditioned reinforcer. These associative effects, establishing non-nicotine stimuli as conditioned stimuli with discriminative stimulus and conditioned reinforcing properties as well as establishing nicotine as a CS, are predicted by basic conditioning principles. However, nicotine can also act non-associatively. Nicotine directly enhances the reinforcing efficacy of other reinforcing stimuli in the environment, an effect that does not require a temporal or predictive relationship between nicotine and either the stimulus or the behavior. Hence, the reinforcing actions of nicotine stem both from the primary reinforcing actions of the drug (and the subsequent associative learning effects) as well as the reinforcement enhancement action of nicotine which is non-associative in nature. Gaining a better understanding of how nicotine impacts behavior will allow for maximally effective tobacco control efforts aimed at reducing the harm associated with tobacco use by reducing and/or treating its addictiveness.

Interoception and learning: import to understanding and treating diseases and psychopathologies

Chemotherapeutic agents nauseate cancer patients. Some individuals with schizophrenia hear voices. Chronic pain can be reduced by analgesics. Nausea, voices, and pain are examples of internal (interoceptive) stimuli closely linked with a disease and/or its treatment. There is evidence that the perception and, hence, role of these internal stimuli can be modified by one's learning history. There is also increased awareness by researchers and practitioners of the potential import of learning involving internal states to some diseases and psychopathologies. Unfortunately, the science, theory, and practice appear to be trailing behind awareness. In this mini-review, we describe two examples: smoking and panic disorder. While doing so, we discuss the need to develop translationally relevant animal models that will allow investigators to better understand the behavioral and neural mechanisms underlying interoception and learning.

Interoceptive conditioning with a nicotine stimulus is susceptible to reinforcer devaluation

Pavlovian conditioning processes contribute to the etiology of nicotine dependence. Conditioning involving interoceptive stimuli is increasingly recognized as playing a role in many diseases and psychopathologies, including drug addiction. Previous animal research on diminishing the influence of interoceptive conditioning has been limited to antagonism and nonreinforced exposures to the drug stimulus. The goal of the present research was to determine whether interoceptive conditioning with a nicotine stimulus could be diminished through an unconditioned stimulus (US) devaluation procedure. In two separate experiments, male Sprague-Dawley rats received nicotine injections (0.4 mg base/kg) followed by intermittent sucrose (26%) access in a conditioning chamber. On intermixed saline sessions, sucrose was withheld. Conditioning was demonstrated by a reliable increase in head entries in the dipper receptacle on nicotine versus saline sessions. After conditioning, rats in a devaluation condition were given access to sucrose in their home cages immediately followed by a lithium chloride (LiCl) injection on 3 consecutive days. On subsequent test days, nicotine-evoked conditioned responding was significantly attenuated. Within-subject (Experiment 1) and between-subjects (Experiment 2) controls revealed that the diminished responding was not attributable to mere exposure to the sucrose US in the devaluation phase. Experiment 2 included a LiCl-alone control group. Repeated illness induced by LiCl did not reduce later nicotine-evoked responding. These findings suggest that there is a direct association between the interoceptive stimulus effects of nicotine and the appetitive sucrose US (i.e., stimulus-stimulus) rather than a stimulus-response association.

Disentangling the nature of the nicotine stimulus

Learning involving interoceptive stimuli likely plays an important role in many diseases and psychopathologies. Within this area, there has been extensive research investigating the interoceptive stimulus effects of abused drugs. In this pursuit, behavioral pharmacologists have taken advantage of what is known about learning processes and adapted the techniques to investigate the behavioral and receptor mechanisms of drug stimuli. Of particular interest is the nicotine stimulus and the use of the two-lever operant drug discrimination task and the Pavlovian drug discriminated goal-tracking task. There is strong concordance between the two methods when using "standard" testing protocols that minimize learning on test days. For example, ABT-418, nornicotine, and varenicline all fully evoked nicotine-appropriate responding. Notably, research from our laboratory with the discriminated goal-tracking task has used an alternative testing protocol. This protocol assesses stimulus substitution based on how well extinction learning using a non-nicotine ligand transfers back to the nicotine stimulus. These findings challenge conclusions based on more "standard" testing procedures (e.g., ABT-418 is not nicotine-like). As a starting point, we propose Thurstone scaling as a quantitative method for more precisely comparing transfer of extinction across doses, experiments, and investigators. We close with a discussion of future research directions and potential implications of the research for understanding interoceptive stimuli.

Conditioned response evoked by nicotine conditioned stimulus preferentially induces c-Fos expression in medial regions of caudate-putamen

Nicotine has both unconditioned and conditioned stimulus properties. Conditioned stimulus properties of nicotine may contribute to the tenacity of nicotine addiction. The purpose of this experiment was to use neurohistochemical analysis of rapidly developing c-Fos protein to elucidate neurobiological loci involved in the processing of nicotine as an interoceptive conditioned stimulus (CS). Rats were injected (SC) in an intermixed fashion with saline or nicotine (16 sessions of each) and placed in conditioning chambers where they were given one of the three conditions depending on group assignment: (a) nicotine paired 100% of the time with intermittent access to sucrose (nicotine-CS condition), (b) nicotine and saline each paired 50% of the time with sucrose (chamber-CS condition), or (c) no sucrose US control (CS-alone condition). Rats in the nicotine-CS condition acquired the discrimination as evidenced by goal-tracking (ie, increased dipper entries before initial sucrose delivery) only on nicotine sessions. The chamber-CS condition showed goal-tracking on all sessions; no goal-tracking was seen in the CS-alone condition. On the test day, rats in each condition were challenged with saline or nicotine and later assessed for c-Fos immunoreactivity. In concordance with previous reports, nicotine induced c-Fos expression in the majority of areas tested; however, learning-dependent expression was specific to dorsomedial and ventromedial regions of caudate-putamen (dmCPu, vmCPu). Only rats in the nicotine-CS condition, when challenged with nicotine, had higher c-Fos expression in the dmCPu and vmCPu. These results suggest that medial areas of CPu involved in excitatory conditioning with an appetitive nicotine CS.

Excitatory conditioning to the interoceptive nicotine stimulus blocks subsequent conditioning to an exteroceptive light stimulus

Previous research has shown that a nicotine conditional stimulus (CS) can compete with (i.e., overshadow) a brief light CS. Another form of competition, blocking, has not yet been examined with the nicotine CS. Groups of rats were assigned to an element training condition. For the N+ group, during each daily 2h element training session, there were ten intravenous nicotine infusions (0.03 mg/kg) followed 30s later with 4s access to sucrose. In the N- group, nicotine and sucrose presentations were explicitly unpaired. The chamber alone group (C alone) had no stimulus presentations. Element training was followed by compound training in all groups. A 30-s houselight was included during the time between the nicotine infusion and paired sucrose delivery. Non-reinforced element presentations assessed relative control of the goal tracking conditioned response (CR). The N+ group showed a higher proportion of CR control by the nicotine than the light. The opposite pattern was found in the N- and C alone groups indicating that nicotine CS controlled less of the CR than the light. Thus, excitatory conditioning with the nicotine CS blocked later conditioning to the light. This finding adds to literature examining the interaction between interoceptive drug CSs and other environmental stimuli.

Nicotine trained as a negative feature passes the retardation-of-acquisition and summation tests of a conditioned inhibitor

Nicotine functions as a negative feature in a Pavlovian discriminated goal-tracking task. Whether withholding of responding to the conditional stimulus (CS) reflects nicotine functioning as a conditioned inhibitor is unknown. Accordingly, the present research sought to determine whether nicotine trained as a negative feature passed the retardation-of-acquisition and summation tests, thus characterizing it as a pharmacological (interoceptive) conditioned inhibitor. In the retardation test, rats received either nicotine (0.4 mg/kg) or chlordiazepoxide (5 mg/kg) negative feature training in which the drug state signaled when a 15-sec light CS would not be paired with sucrose; light was paired with sucrose on intermixed saline sessions. Following acquisition of the discrimination, both groups received nicotine CS training in which sucrose was intermittently available on nicotine but not intermixed saline sessions. Acquisition of conditioned responding to the nicotine CS was slower in the nicotine negative feature group than in the chlordiazepoxide negative feature group. In the summation test, rats were assigned to either the nicotine negative feature group or a pseudoconditioning control. In this control, the light CS was paired with sucrose on half the nicotine and half the saline sessions. Both groups also received excitatory training in which a white noise CS was paired with sucrose. The summation test consisted of presenting the white noise in conjunction with nicotine. Conditioned responding evoked by the white noise was decreased in the negative feature but not the pseudoconditioning group. Combined, the results provide the first evidence that an interoceptive stimulus (nicotine) can become a conditioned inhibitor.

Nicotine competes with a visual stimulus for control of conditioned responding

Environmental stimuli that co-occur with tobacco use come to evoke drug-related conditioned responses (CRs) that appear involved in continued use of nicotine-containing products. In rats, nicotine can serve as a conditional stimulus (CS) for non-drug unconditioned stimuli (USs), prompting the question of whether the nicotine CS can compete with, or overshadow, a non-drug environmental stimulus for control of a CR. In Experiment 1, male Sprague-Dawley rats were assigned to a group [0, 0.01, 0.03, 0.045, or 0.06 mg nicotine (base)/kg/infusion]. During each session, there were 10 intravenous infusions followed by a 30-second houselight to form a compound CS. At light offset there was 4-second access to sucrose. For Experiment 2, groups were nicotine (0.03 mg/kg/infusion) + light compound paired, nicotine + light compound unpaired, nicotine paired and light unpaired, and nicotine unpaired and light paired. Paired stimuli were presented with sucrose similar to Experiment 1. Unpaired stimuli were temporally separated from sucrose. Following acquisition, tests of nicotine and light alone were conducted by intermixing non-reinforced trails into training sessions. Nicotine dose-dependently overshadowed the light CS as shown by reduced light control of conditioned responding with higher doses. The nicotine, light, and nicotine + light compound had to be paired with sucrose to evoke a CR. These results demonstrate nicotine overshadows an exteroceptive visual stimulus. Because exteroceptive stimuli are often the focus of cue-exposure therapy, such competition may help begin to explain the marginal effectiveness of these therapies.

An investigation of bupropion substitution for the interoceptive stimulus effects of nicotine

Although the exact mechanism that makes bupropion hydrochloride (Zyban) effective as a smoking cessation aid has not been fully elucidated, studies have found that bupropion and nicotine share behavioural and neurophysiological properties suggesting that bupropion might serve as a substitute for nicotine. In fact, bupropion prompts nicotine-appropriate responding in operant and Pavlovian drug discrimination studies with rats. A majority of the literature examining this substitution pattern has been done with an operant paradigm. The present research extended this literature by further characterising the behavioural and neuropharmacological properties underlying the substitution for a nicotine conditioned stimulus (CS). Examination of the dose-effect function and temporal dynamics of this substitution pattern showed that bupropion (20 mg/kg) produced conditioned responding similar to nicotine (0.4 mg base/kg) (ED(50) = 9.9 mg/kg) at 15 and 30 min after injection and partially substituted 5 and 60 min post-injection. Bupropion produced a pattern of conditioned responding similar to nicotine during a 60-min extinction test. Additionally, it has been hypothesised that bupropion and nicotine have an overlapping dopaminergic mechanism. We tested the effects of bupropion pretreatment, the nicotine dose-effect function and the ability of dopamine antagonist to block the substitution of bupropion for nicotine. Pretreatment with doses of bupropion that did not substitute for the nicotine stimulus (5 and 10 mg/kg) did not affect nicotine-conditioned responding; pretreatment with 20 mg/kg attenuated nicotine-evoked responding. Pretreatment with the dopamine antagonists SCH-23390 and eticlopride blocked the substitution. Finally, S,S-hydroxybupropion, the major metabolite of bupropion in humans, did not substitute for the nicotine CS.

Nicotine elicits methamphetamine-seeking in rats previously administered nicotine

Research has indicated a high correlation between psychostimulant use and tobacco cigarette smoking in human substance abusers. The objective of the current study was to examine the effects of acute and repeated nicotine administration on responding for intravenous methamphetamine (0.03 mg/kg/infusion) in a rodent model of self-administration, as well as the potential of nicotine to induce reinstatement of previously extinguished drug-taking behavior in male Sprague-Dawley rats. In addition, it was assessed whether nicotine-induced reinstatement of methamphetamine-seeking behavior and nicotine-induced locomotor sensitization require that nicotine be temporally paired with the methamphetamine self-administration session or the locomotor activity chamber. Nicotine acutely decreased methamphetamine self-administration, but did not persistently alter responding during the maintenance of methamphetamine self-administration. However, following extinction of methamphetamine self-administration, nicotine administration reinstated methamphetamine-seeking behavior only in rats that had previously been administered nicotine. Nicotine-induced reinstatement and expression of locomotor sensitization were not dependent on a temporal pairing of nicotine with either the methamphetamine self-administration session or the locomotor activity chamber, respectively. These results indicate that nicotine may be acting, at least in part, through a non-associative mechanism to reinstate methamphetamine-seeking behavior.

Mecamylamine, dihydro-beta-erythroidine, and dextromethorphan block conditioned responding evoked by the conditional stimulus effects of nicotine

Current smokers express the desire to quit. However, the majority find it difficult to remain abstinent. As such, research efforts continually seek to develop more effective treatment. One such area of research involves the interoceptive stimulus effects of nicotine as either a discriminative stimulus in an operant drug discrimination task, or more recently as a conditional stimulus (CS) in a discriminated goal-tracking task. The present work investigated the potential role nicotinic acetylcholine receptors play in the CS effects of nicotine (0.4mg/kg) using antagonists with differential selectivity for beta2*, alpha7*, alpha6beta2*, and alpha3beta4* receptors. Methyllycaconitine (MLA) had no effect on nicotine-evoked conditioned responding. Mecamylamine and dihydro-beta-erythroidine (DHbetaE) dose-dependently blocked responding evoked by the nicotine CS. In a time-course assessment of mecamylamine and DHbetaE, each blocked conditioned responding when given 5min before testing and still blocked conditioned responding when administered 200min before testing. Two novel bis-picolinium analogs (N, N'-(3, 3'-(dodecan-1,12-diyl)-bis-picolinium dibromide [bPiDDB], and N, N'-(decan-1,10-diyl)-bis-picolinium diiodide [bPiDI]) did not block nicotine-evoked conditioned responding. Finally, pretreatment with low dose combinations of mecamylamine, dextromethorphan, and/or bupropion was used to target alpha3beta4* receptors. No combination blocked conditioned responding evoked by the training dose of nicotine. However, a combination of mecamylamine and dextromethorphan partially blocked nicotine-evoked conditioned responding to a lower dose of nicotine (0.1mg/kg). These results indicate that beta2* and potentially alpha3beta4* nicotinic acetylcholine receptors play a role in the CS effects of nicotine and are potential targets for the development of nicotine cessation aids.

Neuropharmacology of the interoceptive stimulus properties of nicotine

Preclinical drug discrimination techniques play a significant role in advancing our knowledge of the receptor mechanisms underlying the interoceptive effects of nicotine. Early reports confirmed that nicotinic acetylcholine receptors (nAChRs) are critical for transduction of the nicotine cue. In recent years, advances in molecular biology and the discovery of novel ligands with greater selectively for specific nAChR subtypes have furthered our understanding of these mechanisms. There is now evidence regarding the specific nAChR subtypes involved in nicotine discrimination; in addition, there is also evidence suggesting that other systems (i.e., adenosine, cannabinoid, dopamine, glutamate and serotonin) may play a modulatory role. The neuroanatomical structures mediating the nicotine cue have also begun to be elucidated. However, much remains to be learned about the predictive validity of the drug discrimination procedure, particularly with regard to the relation between interoceptive and reinforcing effects and individual differences in vulnerability to tobacco dependence. Recent data also suggests that the mechanisms involved in the conditional and discriminative stimulus properties of nicotine may be dissociable. Avenues for future research should include assessing the mechanisms of the subjective effects of nicotine withdrawal, factors contributing to individual differences in sensitivity to the nicotine cue, and the role of behavioral factors involved in drug cross-substitution.

Pavlovian drug discrimination with bupropion as a feature positive occasion setter: substitution by methamphetamine and nicotine, but not cocaine

Bupropion can serve as a discriminative stimulus (S(D)) in an operant drug discrimination task, and a variety of stimulants substitute for the bupropion S(D). There are no reports, however, of bupropion functioning as a Pavlovian occasion setter (i.e. feature positive modulator). The present experiment seeks to fill this gap in the literature by training bupropion in rats as a feature positive modulator that disambiguates when a light will be paired with sucrose. Specifically, on bupropion (10 mg/kg intraperitoneal) sessions, offset of 15-second cue lights were followed by brief delivery of liquid sucrose; saline sessions were similar except no sucrose was available. Rats readily acquired the discrimination with more conditioned responding to the light on bupropion sessions. Bupropion is approved for use as a smoking cessation aid, and more recently has drawn attention as a potential pharmacotherapy for cocaine and methamphetamine abuse. Accordingly, after discrimination training, we tested the ability of cocaine (1-10 mg/kg), methamphetamine (0.1 to 1 mg/kg) and nicotine (0.00625 to 0.2 mg/kg) to substitute for the bupropion feature. Nicotine (0.05 mg/kg) and methamphetamine (0.3 mg/kg) substituted fully for bupropion; cocaine did not substitute. These results extend previous research on shared stimulus properties between bupropion and other stimulants to a Pavlovian occasion setting function. Further, this is the first report of nicotine and methamphetamine substitution for bupropion. The overlap in stimulus properties might explain the effectiveness of bupropion as a smoking cessation aid and highlight the possible utility of bupropion for treatment of stimulant use disorder.

Intravenous nicotine conditions a place preference in rats using an unbiased design

The rewarding effects of nicotine contribute to the chronic use of tobacco products. The place conditioning task, a widely used pre-clinical model to study drug reward, has lead to mixed results in rats when nicotine was administered subcutaneously or intraperitoneally; intravenously administered nicotine has not been examined. Further, much of the research demonstrating a nicotine-conditioned place preference in rats has used a biased design making these results susceptible to non-reward interpretations. The present study assessed whether intravenous (IV) nicotine would condition a place preference in an unbiased design and evaluated important behavioral parameters: nicotine dose, number of conditioning trials, and infusion-to-placement interval. In adult male Sprague Dawley rats, IV nicotine (0.03 mg/kg) conditioned a place preference after 8 conditioning trials. This conditioned preference was observed whether nicotine was infused 10 min before or immediately after placement in the paired environment for 10 min; infusing nicotine immediately after removal from the paired environment did not condition a preference after 4 or 8 conditioning trials. Four conditioning trials were not sufficient to condition a preference regardless of the temporal relation between the paired environment and 0.03 mg/kg nicotine. A 0.01 mg/kg dose of nicotine did not condition a place preference after 4 or 8 trials when infused immediately upon placement in the paired environment. Intravenous nicotine (0.03 mg/kg) has rewarding effects in an unbiased design suggesting that the place conditioning protocol used in the present study might be an especially useful model for studying the processes underlying the conditioned rewarding effects of nicotine.