Development of a nicotine aerosol self-administration model in rats and the effects of e-liquid flavors

Electronic nicotine delivery system (ENDS) use is maintained by the positive reinforcement associated with preferred flavors. These flavors become conditioned reinforcers through pairings with primary reinforcers. This study sought to extend prior research with intravenous nicotine self-administration and develop a more ecologically valid preclinical model of aerosol self-administration in rats that incorporated flavors paired with sucrose. Rats were first trained to respond for oral sucrose with or without raspberry flavor to establish the flavor as a conditioned reinforcer for some groups. Rats were then exposed to aerosol self-administration. All groups responded for raspberry-flavored aerosol with or without nicotine. Rats responded more for raspberry flavored sucrose than unflavored sucrose. Despite raspberry increasing responding for sucrose, the flavor did not function as a conditioned reinforcer during aerosol self-administration and did not increase responding for nicotine. Throughout the aerosol self-administration phase, most groups responded more on the active than inactive lever, and some groups increased their response when the fixed ratio value was increased. At the end of the study, rats in nicotine groups earned similar or fewer aerosol deliveries than rats in vehicle groups. Aerosolized nicotine did not function as a reinforcer in this study, whereas aerosolized raspberry flavor may have maintained self-administration. Further preclinical investigation is needed to articulate the impact of flavors on ENDS use and whether they offset some aversive effects of nicotine or maintain responding on their own. If flavors reduce some aversive effects of self-administered nicotine, then policies to regulate flavors in e-liquids are prudent.

Nicotine Self-Administration With Tobacco Flavor Additives in Male Rats

INTRODUCTION

Nicotine can robustly increase responding for conditioned reinforcers (CRs), stimuli that acquire reinforcing properties based on association with primary reinforcers. Menthol and licorice are tobacco flavoring agents also found in sweet foods (eg, candy and ice cream), making them putative CRs before they are consumed in tobacco. We sought to determine if intravenous self-administration (IVSA) of nicotine was enhanced by the inclusion of oral tobacco flavor CRs.

METHODS

Menthol (160 or 320 µM) or licorice root extract (0.1% or 1%) were established as CRs (paired with 20% sucrose) or "neutral" stimuli (paired with water) in separate groups. During subsequent IVSA tests, nicotine was delivered in conjunction with oral presentations of the CR.

RESULTS

In experiment 1, a menthol CR significantly shifted the peak nicotine dose from 15 µg/kg/infusion (Neutral group) to 3.25 µg/kg/infusion (CR group). In experiment 2, a menthol CR significantly increased operant licks for nicotine (3 µg/kg/infusion) relative to control groups. In experiment 3, both licorice and menthol CRs significantly increased operant licks for nicotine (7.5 µg/kg/infusion) relative to an "inactive" sipper. The licorice CR increased nicotine IVSA in proportion to the strength of the flavor, but both menthol concentrations increased nicotine IVSA to a similar extent.

CONCLUSION

Tobacco flavor additives with conditioned reinforcing properties promote acquisition of nicotine self-administration at low unit doses and may have robust impact on tobacco consumption when nicotine yield is low.

IMPLICATIONS

Tobacco flavor additives are found in rewarding foods (eg, ice cream) and gain palatability based on associations with primary rewards (eg, sugar) making them "conditioned reinforcers." Nicotine increases the motivation for flavor conditioned reinforcers and the present studies show that tobacco flavor additives can interact with nicotine to promote more nicotine self-administration. The interaction between flavors additives and nicotine may promote nicotine exposure and subsequently dependence.

Intravenous and oral caffeine self-administration in rats

Caffeine is widely consumed for its psychoactive effects worldwide. No pre-clinical study has established reliable caffeine self-administration, but we found that caffeine can enhance the reinforcing effects of non-drug rewards. The goal of the present studies was to determine if this effect of caffeine could result in reliable caffeine self-administration. In 2 experiments rats could make an operant response for caffeine delivered in conjunction with an oral 'vehicle' including saccharin (0.2% w/v) as a primary reinforcer. In Experiment 1, intravenous (IV) caffeine infusions were delivered in conjunction with oral saccharin for meeting the schedule of reinforcement. In control conditions, oral saccharin alone or presentations of IV caffeine alone served as the reinforcer. In Experiment 2, access to caffeine was provided in an oral vehicle containing water, decaffeinated instant coffee (0.5% w/v), or decaffeinated coffee and saccharin (0.2%). The concentration of oral caffeine was then manipulated across testing sessions. Oral and IV caffeine robustly increased responding for saccharin in a manner that was repeatable, reliable, and systematically related to unit IV dose. However, the relationship between oral caffeine dose and operant behavior was less systematic; the rats appeared to titrate their caffeine intake by reducing the consummatory response (drinking) rather than the appetitive response (lever pressing). These studies establish reliable volitional caffeine self-administration in rats. The reinforcement enhancing effects of caffeine may help to explain widespread caffeine use by humans, who ingest caffeine in complex vehicles with reinforcing properties.

Orbitofrontal participation in sign- and goal-tracking conditioned responses: Effects of nicotine

Pavlovian conditioned stimuli can acquire incentive motivational properties, and this phenomenon can be measured in animals using Pavlovian conditioned approach behavior. Drugs of abuse can influence the expression of this behavior, and nicotine in particular exhibits incentive amplifying effects. Both conditioned approach behavior and drug abuse rely on overlapping corticolimbic circuitry. We hypothesize that the orbitofrontal cortex (OFC) regulates conditioned approach, and that one site of nicotine action is in the OFC where it reduces cortical output. To test this, we repeatedly exposed rats to 0.4 mg/kg nicotine (s.c.) during training and then pharmacologically inactivated the lateral OFC or performed in vivo electrophysiological recordings of lateral OFC neurons in the presence or absence of nicotine. In Experiment 1, animals were trained in a Pavlovian conditioning paradigm and behavior was evaluated after inactivation of the OFC by microinfusion of the GABA agonists baclofen and muscimol. In Experiment 2, we monitored phasic firing of OFC neurons during Pavlovian conditioning sessions. Nicotine reliably enhanced conditioned responding to the conditioned cue, and inactivation of the OFC reduced conditioned responding, especially the sign-tracking response. OFC neurons exhibited phasic excitations to cue presentation and during goal tracking, and nicotine acutely blunted this phasic neuronal firing. When nicotine was withheld, both conditioned responding and phasic firing in the OFC returned to the level of controls. These results suggest that the OFC is recruited for the expression of conditioned responses, and that nicotine acutely influences this behavior by reducing phasic firing in the OFC.

Extending the Role of Associative Learning Processes in Nicotine Addiction

Compulsive smoking is a worldwide public health problem. Although research has confirmed the importance of associative learning processes in nicotine addiction, therapies targeting nicotine-associated cues still have a high relapse rate. Most theories conceptualize nicotine as an ‘outcome’ that reinforces behaviors and/or changes the affective value of stimuli. Albeit important, this view does not capture the complexity of associative processes involved in nicotine addiction. For example, nicotine serves as a conditional stimulus acquiring new appetitive/affective properties when paired with a non-drug reward. Also, nicotine functions as an occasion setter that participates in higher-order associative processes that likely permit a more pervasive influence of conditioned cues that are resistant to typically cue-exposure therapy techniques. Finally, nicotine appears to amplify the salience of other stimuli that have some incentive value resulting in enhanced nicotine selfadministration and conditioned reinforcement processes. Future smoking intervention strategies should take into consideration these additional associative learning processes.

Differentiating the primary reinforcing and reinforcement-enhancing effects of varenicline

RATIONALE

Varenicline (VAR), a smoking cessation aid that is a partial agonist at nicotinic receptors, mimics the reinforcement-enhancing effects of nicotine. Varenicline, when accompanied by non-drug cues, is self-administered by rats, though it is unclear whether this results from varenicline acting as a primary reinforcer or a reinforcement enhancer of the cues.

OBJECTIVES

This study sought to disentangle these two potential actions.

METHODS

Rats were allowed to self-administer intravenous nicotine, saline, or varenicline during 1-h sessions in operant chambers equipped with two levers. Five groups had concurrent access to drug infusions and a moderately reinforcing visual stimulus (VS) for responding on separate levers. Meeting the reinforcement schedule on one lever was reinforced with VAR (0.01, 0.06, 0.1 mg/kg/infusion), nicotine (0.06 mg/kg/infusion), or saline, while meeting the same schedule on the other lever delivered the VS. Additional groups were reinforced for pressing a single "active" lever and received VAR paired with the VS, the VS with response-independent infusions of VAR, or VAR alone (0.1 mg/kg/infusion).

RESULTS

Rats readily responded for VAR paired with VS on a single lever. However, when VAR was the only reinforcer contingent on a response, rats did not respond more than for saline.

CONCLUSIONS

These findings show that VAR does not serve as a primary reinforcer in rats at doses that increase responding for non-drug reinforcers. These data are consistent with research showing that the primary reinforcing effects of VAR are weak, at best, and that the primary reinforcing and reinforcement-enhancing actions of nicotinic drugs are pharmacologically distinct.

The incentive amplifying effects of nicotine are reduced by selective and non-selective dopamine antagonists in rats

Nicotine is a psychomotor stimulant with 'reinforcement enhancing' effects--the actions of nicotine in the brain increase responding for non-nicotine rewards. We hypothesized that this latter effect of nicotine depends on increased incentive properties of anticipatory cues; consistent with this hypothesis, multiple laboratories have reported that nicotine increases sign tracking, i.e. approach to a conditioned stimulus (CS), in Pavlovian conditioned-approach tasks. Incentive motivation and sign tracking are mediated by mesolimbic dopamine (DA) transmission and nicotine facilitates mesolimbic DA release. Therefore, we hypothesized that the incentive-promoting effects of nicotine would be impaired by DA antagonists. To test this hypothesis, separate groups of rats were injected with nicotine (0.4mg/kg base) or saline prior to Pavlovian conditioning sessions in which a CS (30s illumination of a light or presentation of a lever) was immediately followed by a sweet reward delivered in an adjacent location. Both saline and nicotine pretreated rats exhibited similar levels of conditioned approach to the reward location (goal tracking), but nicotine pretreatment significantly increased approach to the CS (sign tracking), regardless of type (lever or light). The DAD1 antagonist SCH-23390 and the DAD2/3 antagonist eticlopride reduced conditioned approach in all rats, but specifically reduced goal tracking in the saline pretreated rats and sign tracking in the nicotine pretreated rats. The non-selective DA antagonist flupenthixol reduced sign-tracking in nicotine rats at all doses tested; however, only the highest dose of flupenthixol reduced goal tracking in both nicotine and saline groups. The reductions in conditioned approach behavior, especially those by SCH-23390, were dissociated from simple motor suppressant effects of the antagonists. These experiments are the first to investigate the effects of dopaminergic drugs on the facilitation of sign-tracking engendered by nicotine and they implicate dopaminergic systems both in conditioned approach as well as the incentive-promoting effects of nicotine.

Effects of nicotine on olfactogustatory incentives: preference, palatability, and operant choice tests

INTRODUCTION

The use of additives in tobacco may capitalize on the incentive motivational properties of tastes and scents such as mint (menthol), vanilla, and strawberry. These incentives are intended to increase tobacco experimentation, but their salience may also be enhanced by the incentive amplifying effects of nicotine (NIC). The goal of the present studies was to investigate the potential interaction between the incentive amplifying effects of NIC and gustatory incentives.

METHODS

One of two discriminable tastes (grape or cherry Kool-Aid®; 0.05% wt/vol; unsweetened) was paired with sucrose (20% wt/vol; conditioned stimulus [CS+]) in deionized water, whereas the other taste (CS-) was presented in deionized water. Experiment 1 investigated the effects of NIC pretreatment on preference for the CS+ versus CS- in 2-bottle choice tests. Experiment 2 investigated the effects of NIC on palatability of the CS+ and CS- using orofacial taste reactions. Experiment 3 investigated the effects of NIC on reinforcement by the CS+ and CS- using a concurrent choice operant task.

RESULTS

NIC pretreatment robustly increased operant responding for the CS+ but did not alter responding for the CS- in the operant choice task (Experiment 3). However, NIC pretreatment did not alter intake or palatability of the CS+ or CS- (Experiments 1 and 2).

CONCLUSIONS

NIC increases the reinforcing effects of gustatory incentive stimuli, even though these stimuli were not paired with NIC administration. The findings suggest that adding taste incentives to tobacco products may increase the attractiveness of these products to consumers and the probability of repeated use.

The effect of nicotine on sign-tracking and goal-tracking in a Pavlovian conditioned approach paradigm in rats

RATIONALE

Nicotine (NIC) potently increases operant responding for non-NIC reinforcers, and this effect may depend on drug-mediated increases in incentive motivation. According to this hypothesis, NIC should also potently increase approach to Pavlovian-conditioned stimuli associated with rewards.

OBJECTIVE

The present studies explored the effects of NIC on Pavlovian-conditioned approach responses.

METHOD

To do so, liquid dippers were used to deliver an unconditioned stimulus (US; 0.1 ml sucrose) after presentation of a conditioned stimulus (CS; 30 s illumination of a stimulus light)-both the CS and US were presented in receptacles equipped to monitor head entries.

RESULTS

In experiment 1, the CS and US were presented in the same receptacle, but NIC pretreatment (0.4 mg/kg base) did not increase conditioned approach responses. Delivery of the sucrose US was then shifted to receptacle in a different location. All rats learned to approach the new US location (goal-tracking) at similar rates. Approach to the CS receptacle (sign-tracking) declined for saline-pretreated rats, but NIC pretreatment increased sign-tracking. In experiment 2, NIC pretreatment increased sign-tracking when the CS and US were spatially separated during acquisition. In experiment 3, NIC pretreatments were replaced with saline, but the effect of NIC persisted for an additional 24 test sessions.

CONCLUSION

The findings suggest that NIC increases incentive motivation and that this effect is long-lasting, persisting beyond the pharmacological effects of NIC.

Caffeine increases the motivation to obtain non-drug reinforcers in rats

BACKGROUND

Caffeine is widely considered to be a reinforcer in humans, but this effect is difficult to measure in non-human animals. We hypothesized that caffeine may have dual reinforcing effects comparable to nicotine--limited primary reinforcing effects, but potent reinforcement enhancing effects. The present studies tested this hypothesis by investigating the effect of caffeine on responding for non-drug rewards.

METHODS

In two experiments, rats were shaped to respond on a progressive ratio (PR) schedule for sucrose solution (20%, w/v; experiment 1) or a fixed ratio 2 (FR2) schedule for a moderately reinforcing visual stimulus (VS; experiment 2). Pretreatment with various doses of caffeine (0-50 mg/kg, intraperitoneal injection) were administered prior to tests over successive week days (M-F). In experiment 1, acute administration of low-moderate caffeine doses (6.25-25 mg/kg) increased responding for sucrose under the PR schedule. This effect of caffeine declined over the initial 15 test days. In experiment 2, only acute pretreatment with 12.5mg/kg caffeine increased responding for the visual stimulus and complete tolerance to this effect of caffeine was observed over the 15 days of testing. In follow up tests we found that abstinence periods of 4 and 8 days resulted in incomplete recovery of the enhancing effects of caffeine.

CONCLUSION

The findings suggest that caffeine enhances the reinforcing effects of non-drug stimuli, but that the pharmacological profile of these effects may differ from other psychomotor stimulants.

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

INTRODUCTION

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

The role of conditioning history and reinforcer strength in the reinforcement enhancing effects of nicotine in rats

RATIONALE

Nicotine (NIC) administration can increase behaviors that result in delivery of non-drug reinforcers (e.g., salient sensory stimuli). However, little is known about the circumstances under which NIC increases these behaviors.

OBJECTIVE

The present studies sought to extend the reinforcement enhancing effects of NIC to sucrose rewards for which intensity could be systematically manipulated.

METHOD

In Experiment 1, rats were trained to respond for sucrose (30% w/v) on a progressive ratio (PR) schedule of reinforcement and were pretreated with NIC (0.4 mg/kg free-base) or physiological saline (SAL). The intensity of the sucrose reward was manipulated over subsequent testing sessions (0-60% w/v). Similar procedures were used in Experiment 2; however, each subject received only one sucrose concentration (0-20%) to control for conditioning history. In Experiment 3, a fixed ratio 3 (FR3) schedule of reinforcement was used to investigate putative activating effects of NIC. Experiment 4 investigated whether NIC pretreatment would reduce sucrose intake in limited-access drinks.

RESULTS

In Experiment 1, NIC increased the motivation to obtain all sucrose concentrations, including water. However, when conditioning history was controlled (Experiment 2) the reinforcement enhancing effects of NIC were systematically related to the strength of the reinforcer. In Experiment 3, NIC neither increased nor decreased responding for sucrose. In Experiment 4, NIC reduced sucrose intake, but only at concentrations that resulted in peak drink volumes (5-20%).

CONCLUSION

The results suggest that the reinforcement enhancing effects of NIC depend on conditioning history and do not appear to be the result of simple behavioral activation.

Bupropion and nicotine enhance responding for nondrug reinforcers via dissociable pharmacological mechanisms in rats

RATIONALE

Nicotine serves as a primary reinforcer but also potently enhances responding for nonnicotine stimuli with reinforcing properties. One of the most successful pharmacotherapies for smoking cessation, bupropion, also increases responding for nondrug reinforcers such as food and brain stimulation rewards.

OBJECTIVE

The present studies investigated whether treatment with bupropion and nicotine had similar effects on responding for a reinforcing visual stimulus (VS). They also investigated whether the effects of bupropion and nicotine depended on common pharmacological substrates.

RESULTS

Nicotine (0.4 mg/kg base) enhanced responding for the VS, and this enhancing effect increased across testing sessions, replicating our previous findings. Bupropion (3, 10, and 30 mg/kg salt) dose-dependently increased responding for the VS. Treatment with 10 and 30 mg/kg bupropion resulted in a profile similar to nicotine; operant responding increased over repeated drug treatments. The reinforcement enhancing effect of nicotine, but not bupropion, was blocked by pretreatment with the nicotinic acetylcholine receptor antagonist mecamylamine. In contrast, the reinforcement enhancing effect of bupropion, but not nicotine, was blocked by pretreatment with the alpha noradrenergic antagonist prazosin.

CONCLUSION

The reinforcement enhancing effects of nicotine and bupropion increased over time and repeated treatments suggesting a shared mechanism of action. However, the reinforcement enhancing effects of nicotine are mediated by nicotinic acetylcholine receptors, whereas the reinforcement enhancing effects of bupropion were mediated by alpha noradrenergic receptors.

Naltrexone attenuation of conditioned but not primary reinforcement of nicotine in rats

RATIONALE

Opioid neurotransmission has been implicated in reinforcement-related processes for several drugs of abuse, including opiates, stimulants, and alcohol. However, less is known about its role in the motivational effects of nicotine and nicotine-associated environmental cues.

OBJECTIVE

This study investigated whether pretreatment with naltrexone, an opioid receptor antagonist, alters conditioned incentive salience of nicotine cues under two conditions: cue-induced reinstatement of nicotine-seeking after extinction and cue-maintained responding during extinction. The effect of naltrexone on nicotine self-administration during the maintenance phase was also examined.

MATERIALS AND METHODS

Male Sprague-Dawley rats were trained in daily 1-h sessions to self-administer nicotine (0.03 mg/kg/infusion, i.v.) on a fixed-ratio 5 schedule and associate a conditioned stimulus (CS) with each nicotine delivery. Once responding was extinguished by saline substitution for nicotine and omission of the CS, the reinstatement tests were conducted following subcutaneous administration of naltrexone (0, 0.25, 1, 2 mg/kg). In separate groups of rats, naltrexone (0, 2 mg/kg) was chronically given before each extinction sessions, where responses on the active lever resulted in presentations of the CS without nicotine infusion (saline substitution). Self-administration/naltrexone tests were conducted in different groups of rats receiving similar nicotine self-administration training.

RESULTS

Naltrexone significantly attenuated the CS-reinstated responding on the active, previously nicotine-reinforced lever in the reinstatement tests and the CS-maintained active lever responding during the extinction tests. In contrast, neither acute nor chronic naltrexone produced an effect on nicotine self-administration behavior.

CONCLUSIONS

These results indicate that activation of opioid receptors is implicated in mediation of the conditioned incentive properties of nicotine cues but not in the maintenance of nicotine self-administration. Therefore, these findings suggest that opioid receptor antagonists might have clinical potential for prevention of smoking relapse associated with exposure to environmental cues.

The motivation to obtain nicotine-conditioned reinforcers depends on nicotine dose

Stimuli associated with nicotine (NIC) can acquire new meaning via Pavlovian conditioning. If a stimulus is associated with the primary reinforcing effects of NIC, the new conditional properties of the stimulus should make it a more valuable reinforcer (i.e., increase the motivation to obtain the stimulus), and this value should be based, in part, on the strength or intensity of the primary reinforcer (i.e., NIC dose). The purpose of the present study was to investigate whether NIC-conditioned reinforcement increased motivation to obtain non-NIC stimuli, as reflected by performance on a progressive ratio (PR) reinforcement schedule, and whether this increased motivation was systematically related to NIC dose. Two Paired groups were allowed to nose-poke for NIC (0.03 or 0.09mg/kg/infusion, IV) accompanied by 15-s illumination of a stimulus light (conditional stimulus or CS). Two Unpaired groups (0.03 or 0.09mg/kg/infusion) could also make a nose-poke response for the CS; however their NIC infusions were controlled by the Paired group (i.e., yoked design). A fifth group (CS-Only) was allowed to nose-poke for CS presentations and saline infusions. After 29 conditioning sessions the nose-poke operant was prevented by obscuring the receptacle and the CS (accompanied by saline infusion for all groups) was made contingent upon a novel operant response (lever press). During the acquisition of this novel response, each CS/saline infusion earned increased the number of responses required to earn the next CS/saline infusion. Pairings with the primary reinforcing effects of NIC resulted the acquisition of a novel response for the CS. Motivation to obtain the CS depended on salience (dose) of the primary reinforcement (NIC).

Occasion setting by drug states: Functional equivalence following similar training history

Three experiments examined whether a drug state serving as a positive feature for pairings between a discrete conditional stimulus (CS, 15-s light or 15-s noise) and sucrose could transfer facilitative control to a CS with which it had never been presented. To do so, a CS was paired with a sucrose reward in the nicotine (0.4 mg/kg), amphetamine (AMP, 1mg/kg), or chlordiazepoxide (CDP, 5mg/kg) drug state; in separate saline sessions the CS was presented but was not followed by any reward. All three drug states facilitated responding to a discrete CS; previous studies found that this facilitation did not depend on direct associations between the drug state and sucrose. When a second discrimination was trained (e.g., CDP: light-sucrose and nicotine: noise-sucrose) the drug states facilitated responding to the CS trained in that state (nicotine: noise) as well as the CS normally presented in the other drug state (e.g., nicotine: light). A novel drug state (e.g., amphetamine) did not affect responding to either CS, indicating that the originally trained drug states had acquired functional similarity based on learning history. Also, a novel or ambiguous CS did not evoke responding in the previously trained drug state, indicating that both the features (drug states) and target conditional stimuli had to be trained in discriminations before transfer could occur.

Cue-induced reinstatement of nicotine-seeking behavior in rats: effect of bupropion, persistence over repeated tests, and its dependence on training dose

RATIONALE

The motivational effects of nicotine-associated cues have been demonstrated in animal studies. However, it is unknown whether the effectiveness of nicotine cues in reinstating nicotine-seeking varies with the extent of prior nicotine self-administration. In addition, the issue of whether bupropion (an FDA-approved smoking cessation medication) interferes with the conditioned incentive of nicotine cues remains to be addressed.

OBJECTIVE

This study determined the relationship of cue-reinstated nicotine-seeking and the levels of prior self-administration and examined the effect of bupropion on cue-induced reinstatement of nicotine-seeking in comparison with that on self-administration.

MATERIALS AND METHODS

Male Sprague-Dawley rats were trained in daily 1-h sessions to intravenously self-administer nicotine at different doses (0, 0.015, 0.03, 0.06 mg/kg/infusion) and to associate an auditory/visual cue with each nicotine delivery. After extinction, three reinstatement tests at 15 day intervals were conducted with re-presentation of the cue without nicotine delivery. In separate groups of rats trained with 0.03 mg/kg/infusion nicotine, bupropion (0, 10, 20, 40 mg/kg) was intraperitoneally administered to different groups before the reinstatement and in a within-subject design before the self-administration tests.

RESULTS

Cue-induced reinstatement of active lever responses was observed at all nicotine doses in the first reinstatement test, but at only the two highest doses during the second and third tests. The magnitude of reinstatement was positively correlated with level of prior responding for nicotine. Bupropion pretreatment decreased nicotine self-administration but enhanced cue-reinstated nicotine-seeking.

CONCLUSIONS

These results demonstrate the positive correlation of cue-reinstated nicotine-seeking with prior responding for nicotine self-administration and the persistence of the cue effect after taking higher doses of nicotine. The results of pharmacological tests suggest that although it is able to help achieve smoking cessation, bupropion may have little clinical benefit for the prevention of relapse associated with exposure to environmental smoking cues.

Conditioned reinforcement in rats established with self-administered nicotine and enhanced by noncontingent nicotine

RATIONALE

Nicotine is widely assumed to convey reinforcing properties upon tobacco-related stimuli through associative learning. We have proposed that the reinforcement derived from these conditional stimuli can be inflated by a nonassociative "reinforcement-enhancing" effect of nicotine.

OBJECTIVES

Experiment 1 investigated whether nicotine could establish a stimulus as a conditioned reinforcer. Using the same subjects, Experiment 2 examined whether responding for a nicotine-associated stimulus was enhanced by response-independent administration of nicotine.

MATERIALS AND METHODS

Self-administered nicotine (Paired group, 0.03 mg kg(1) infusion(-1)) or saline (conditional stimulus or CS-Only group) was paired with a stimulus light (CS). An Unpaired group, yoked to the Paired group, received equal exposure to nicotine and the CS, but each event was temporally separated. To test for conditioning, the CS was then made contingent upon a novel lever-pressing response. In Experiment 2, a subset of the paired rats (self-administering) continued to lever press while receiving contingent nicotine and the CS. To determine whether nicotine enhanced responding for the CS, two remaining subsets of the Paired group responded for the CS while receiving nicotine (YNIC) or saline (YSAL) yoked to the self-administering rats. All remaining control groups received response-contingent CS presentations, together with yoked nicotine or saline.

RESULTS

Pairing self-administered nicotine with the CS promoted the acquisition of a novel response for the CS. In Experiment 2, the Paired YNIC group responded at higher rates than control groups receiving YNIC or YSAL.

CONCLUSIONS

Nicotine can establish stimuli as conditioned reinforcers for which noncontingent nicotine can enhance responding.

Reinforcement enhancing effect of nicotine and its attenuation by nicotinic antagonists in rats

RATIONALE

Recent studies have demonstrated that nicotine can enhance operant responding for other nonpharmacological reinforcing stimuli. However, the nature of the reinforcement-enhancing effect of nicotine remains largely unknown.

OBJECTIVE

The present study determined the dose dependency of the ability of nicotine to increase lever-pressing responses maintained by a compound visual stimulus (VS) in rats and examined its sensitivity to pharmacological antagonism of nicotinic acetylcholine receptors (nAChRs).

MATERIALS AND METHODS

Male Sprague-Dawley rats were trained in daily 1-h sessions to lever press for delivery of a VS (1 s lever light on and 60 s house light off) on a fixed ratio 5 schedule. During these sessions, eight scheduled response-independent intravenous infusions of nicotine (total amount: 0, 0.06, 0.12, 0.24, 0.48 mg kg(-1) h(-1)) were delivered. In pharmacological tests, a nonselective nAChR antagonist mecamylamine, alpha4beta2-selective antagonist dihydro-beta-erythroidine (DHbetaE), and alpha7-selective antagonist methyllycaconitine (MLA) were administered in different groups of rats 30 min before the session.

RESULTS

The VS maintained a moderate level of lever-pressing responses and nicotine dose-dependently increased responses for the VS presentations. Preteatment of mecamylamine and DHbetaE but not MLA significantly attenuated the nicotine-enhanced responding. However, mecamylamine had no effect on responding for the VS in rats that received scheduled saline infusions.

CONCLUSIONS

These results demonstrate dose dependency of the reinforcement-enhancing effect of nicotine and suggest that activation of the alpha4beta2- but not alpha7-containing nAChRs may mediate this effect.

The reinforcement enhancing effects of nicotine depend on the incentive value of non-drug reinforcers and increase with repeated drug injections

We have hypothesized that nicotine has two effects on reinforcement; it increases the probability of responses resulting in nicotine delivery (primary reinforcement) and enhances the apparent reward value of non-nicotine reinforcers (reinforcement enhancing effect). The present studies investigated two predictions generated by this hypothesis: (1) that the reinforcement enhancing effect will depend on apparent stimulus reward value and (2) that the temporal profile of this effect would depend on the pharmacological profile of nicotine. In Experiment 1, rats were trained to lever press for one of two audio-visual stimuli that differed in their intrinsic reinforcing value and then the effect of pre-session nicotine (0.4 mg/kg base) or saline injections was tested. The stimulus that supported very low rates of operant responding displayed smaller increases in responding after pre-session injections of nicotine. In Experiment 2 the effect of nicotine injected 5 min before the session was compared to the effect of nicotine injected 1h after the session using the more reinforcing stimulus condition from the first experiment. A control group received only vehicle injections. In contrast to nicotine injected just prior to the session, post-session injections of nicotine had no detectable effect on responding for the more reinforcing stimulus. These results indicate that the reinforcement enhancing action of nicotine depends on the intensity of the primary reinforcer and that enhanced reinforcement by nicotine depends on coincident access to a stimulus with reinforcing properties.

The role of nicotinic acetylcholine receptors in the primary reinforcing and reinforcement-enhancing effects of nicotine

The primary reinforcing effects of nicotine are mediated by the drugs action at central nervous system nicotinic acetylcholine receptors (nAChRs). Although previous studies have demonstrated that nicotine potently enhances responding for non-pharmacological stimuli, the role of nAChRs in this reinforcement-enhancing effect is not known. The two reinforcement-related effects of nicotine can be dissociated in a paradigm that provides concurrent access to drug infusions and a non-pharmacological visual stimulus (VS). The present study characterized the role of nAChRs in the primary reinforcing effect of nicotine and the reinforcement-enhancing effect of nicotine. For rats with access to VS (VS-Only), nicotine (NIC-Only), both reinforcers contingent upon one response (NIC+VS) or both reinforcers contingent upon separate responses (2-Lever), unit dose-response relationships (0, 30, 60, or 90 microg/kg/infusion, free base) were determined over a 22-day acquisition period. Expression of the two reinforcement-related effects of nicotine was manipulated by pharmacological antagonism of nAChRs (1 mg/kg mecamylamine, subcutaneous, 5-min before the session) or by substituting saline for nicotine infusions (ie extinction) over a series of seven test sessions. Unit dose manipulations yielded an inverse dose-response relationship for active lever responding in the NIC+VS group. The dose-response relationships for rats with independent access to each reinforcer (2-Lever group) were relatively flat. For the 2-Lever group, acute mecamylamine challenge blocked the reinforcement-enhancing effects of nicotine, VS-lever responding decreased to basal levels on the first day of mecamylamine treatment or saline substitution (to the level of the VS-Only group). In contrast, nicotine-lever responding decreased gradually over the 7-day testing period (similar to saline extinction). The two reinforcement-related effects of nicotine are mediated by nAChRs but can be dissociated by acute and chronic profiles.

The interoceptive Pavlovian stimulus effects of caffeine

The present research sought to test whether caffeine functioned as a Pavlovian cue in two ways--as a positive drug feature or as a conditional stimulus (CS). As a positive feature (Experiment 1), brief light presentations were followed by sucrose only on sessions in which caffeine (10 mg/kg) was administered. On intermixed saline sessions, light presentations were not followed by sucrose. The light came to control robust goal tracking (i.e., conditioned responding) only in caffeine sessions. Thus, caffeine disambiguates when the light was paired with sucrose. Decreasing the dose of caffeine decreased the conditioned responding evoked by the light (ED(50)=4.16 mg/kg). Neither nicotine nor amphetamine substituted for the caffeine feature. As a CS, caffeine (10 or 30 mg/kg, Experiments 2a and 2b, respectively) signaled intermittent access to sucrose--no light presentations. No sucrose or lights were presented on intermixed saline sessions. The caffeine CS, regardless of training dose, acquired the ability to evoke only a weak goal-tracking CR. The nature of this dissociation between caffeine as a drug feature and a CS is discussed within the context of past research finding a similar dissociation with amphetamine and chlordiazepoxide, but not with nicotine.

Self-administered and noncontingent nicotine enhance reinforced operant responding in rats: impact of nicotine dose and reinforcement schedule

RATIONALE

Nicotine infusions that are self-administered (contingent) or response-independent (noncontingent) increase lever pressing for a reinforcing nonpharmacological stimulus in rats, suggesting that in addition to primary reinforcement, nicotine self-administration may result from nicotine enhancing the reinforcement derived from nonnicotine stimuli.

OBJECTIVES

Based on our previous research, in this study, we tested the hypothesis that contingent and noncontingent nicotine would equally elevate responding for a moderately reinforcing visual stimulus, across a range of nicotine doses on both fixed ratio and progressive ratio reinforcement schedules.

MATERIALS AND METHODS

The rats lever pressed for a visual stimulus with contingent nicotine, noncontingent nicotine, or contingent saline. Separate groups responded for saline or nicotine without the visual stimulus. Three doses of nicotine (0.01, 0.03, and 0.09 mg/kg per infusion, free base) were tested in a between-groups design. After responding on an escalating fixed ratio reinforcement schedule, the rats were tested on a progressive ratio schedule.

RESULTS

Compared to responding for the visual stimulus with saline, both contingent and noncontingent nicotine equally elevated lever pressing for the stimulus at each dose on fixed and progressive ratio schedules. In the absence of the stimulus, only the highest nicotine dose sustained self-administration.

CONCLUSIONS

The ability of noncontingent nicotine to elevate responding for a moderately reinforcing visual stimulus occurs across a range of doses, and both self-administered and noncontingent nicotine equally increase motivation to obtain the stimulus, as reflected by performance on a progressive ratio schedule. In the absence of a contingent stimulus, primary reinforcement from nicotine only supports self-administration at high nicotine doses in rats.

Facilitation by drug states does not depend on acquired excitatory strength

Three experiments examined the effects of drug-extinction when a drug state served as a conditional stimulus (CS) for sucrose delivery or as a positive feature for pairings between a discrete CS (e.g., 15-s light-on) and sucrose. Some conditioning models predict that drug state will facilitate the conditional response (CR) based on an association with sucrose whether the drug is trained as a CS or as a facilitator. If so, repeated presentation of the drug state alone (drug-extinction) should decrease the CR in both situations. Nicotine (0.4mg/kg), amphetamine (AMP, 1mg/kg), and chlordiazepoxide (CDP, 5mg/kg) facilitated a goal tracking conditioned response to the discrete CS; however, AMP and CDP did not evoke reliable responding without an interposed stimulus, suggesting that associations between these drug states and sucrose are not expressed as anticipatory food seeking (goal tracking). Repeated presentation of each drug state alone did not disrupt facilitation by nicotine, amphetamine, or CDP; suggesting that the drug states did not facilitate goal tracking based on a direct association with sucrose. This latter finding implicates a higher-order or non-associative mechanism for facilitation of anticipatory food seeking by drug states in this Pavlovian discrimination task.

Operant responding for conditioned and unconditioned reinforcers in rats is differentially enhanced by the primary reinforcing and reinforcement-enhancing effects of nicotine

RATIONALE

Nicotine self-administration in rats is modest when response-contingent nicotine infusions are delivered alone (primary reinforcement) but robust when nicotine infusions are combined with a mildly reinforcing non-pharmacological stimulus. Furthermore, response-independent (non-contingent) nicotine administration also elevates responding for that same non-pharmacological stimulus, suggesting that in addition to primary reinforcement, nicotine can enhance the incentive value of other reinforcers.

OBJECTIVES

In this study, we tested the hypothesis that the reinforcement-enhancing effects of non-contingent nicotine are more dependent on the reinforcing strength of the non-pharmacological stimulus than are the effects of contingent nicotine.

MATERIALS AND METHODS

A weakly reinforcing light-tone stimulus was established as a conditioned reinforcer by repeated pairings with sucrose for some rats, or by delivery in an explicitly unpaired design with sucrose to other rats. Subsequently, both groups lever pressed for the stimulus with contingent nicotine, non-contingent nicotine (0.06 mg kg(-1) per infusion, freebase), or non-contingent saline, according to fixed ratio and progressive ratio reinforcement schedules.

RESULTS

Compared to sucrose-unpaired training, repeated association with sucrose established the light-tone stimulus as a robust conditioned reinforcer. Contingent and non-contingent nicotine equally elevated responding for this conditioned stimulus. Conversely, for the less reinforcing (sucrose-unpaired) stimulus contingent nicotine more effectively elevated behavior compared to non-contingent nicotine.

CONCLUSIONS

The reinforcement-enhancing effect of nicotine increases behavior controlled by both conditioned and unconditioned reinforcers; however, for less salient stimuli associative processes derived from the primary reinforcing effects of contingent nicotine may also be important. These data suggest that nicotine present in tobacco may differentially modulate stimulus-driven behavior in smokers.

Preexposure to nicotine alters the subsequent locomotor stimulant effects of bupropion in rats

Little is known about the interaction between nicotine and bupropion (Zyban), but many studies suggest they have neurological and behavioral similarities. One feature of drugs with common profiles is the ability to cross-sensitize possibly through neurological changes in the reward pathway. Activation of this pathway might explain the effectiveness of bupropion as a smoking cessation aid. The present research investigated whether repeated nicotine administration altered the subsequent locomotor effects of bupropion. In experiment 1, rats were preexposed to nicotine (0.4 mg/kg subcutaneously) or saline on eight separate occasions in the home cage and then tested with bupropion (0, 20, or 30 mg/kg) in locomotor chambers. The acute stimulant effect of 30 mg/kg of bupropion was potentiated by nicotine preexposure. In experiment 2, rats received nicotine repeatedly paired with the locomotor chambers or home cages. An additive effect was observed between acute bupropion and nicotine-conditioned hyperactivity in the chamber-paired group. This enhancement of the acute locomotor effects of bupropion might reflect alterations in common dopaminergic processes.

Characterization of nicotine's ability to serve as a negative feature in a Pavlovian appetitive conditioning task in rats

RATIONALE

Pavlovian feature negative discriminations have been widely used to understand inhibitory conditioning processes using exteroceptive stimuli. Comparatively little is known about inhibitory conditioning processes using a drug state as a negative feature. A negative feature signals that presentation of a conditional stimulus (CS) will not be paired with an unconditioned stimulus.

OBJECTIVES

The present research examined whether nicotine served as a negative feature and started characterizing its properties.

METHODS AND RESULTS

In acquisition, rats received intermixed saline and nicotine (0.4 mg/kg, base) sessions. On saline sessions, a 15-s light CS was paired with 4-s access to sucrose; the CS was presented on nicotine sessions, but sucrose was withheld. The discrimination was acquired with more goal tracking during the CS on saline sessions. Nicotine's inhibition of this conditioned response (CR) was sensitive to nicotine dose (ED50=0.225) and injection to testing interval (CR returned at 200 min). Mecamylamine pretreatment, but not hexamethonium, produced a loss of inhibitory control by nicotine suggesting a role for central nicotinic acetylcholine receptors. Amphetamine, bupropion, arecoline, and chlordiazepoxide, but not caffeine, substituted for the nicotine feature. However, in locomotor tests, amphetamine and bupropion increased activity; arecoline and chlordiazepoxide decreased activity. For this reason, the motor effects of these ligands could not be dissociated from substitution via shared stimulus properties.

CONCLUSIONS

This feature negative task provides a preclinical model for studying how drug states inhibit responding, although identifying the process(es) mediating CR inhibition will require further research.

Complex interactions between nicotine and nonpharmacological stimuli reveal multiple roles for nicotine in reinforcement

RATIONALE

Although considerable progress has been made, we do not yet fully understand the behavioral and neurobiological basis of nicotine reinforcement, and without this knowledge, treatment strategies aimed at reducing smoking remain deficient.

OBJECTIVES

This review describes an original perspective on nicotine reinforcement, which arises from substantial evidence of complex interactions between nicotine and nonpharmacological stimuli. We hypothesize that nicotine reinforcement derives from at least two sources: (1) primary reinforcement, an action that requires response-dependent drug administration and is capable of conveying secondary reinforcing effects on associated stimuli, and (2) the reinforcement-enhancing effect of nicotine, which directly enhances behavior maintained by salient nonnicotine stimuli and does not require a contingent relationship between drug administration and reinforced operant responding. Although novel for nicotine, this hypothesis has origins in an extensive literature on the reinforcing effects of psychostimulants. Empirical support for this hypothesis, based largely on animal models of reinforcement, will be presented.

CONCLUSIONS

Animal models of drug reinforcement have evolved to reflect our growing awareness of the multidimensional nature of drug dependence in humans. Investigating the interaction between nicotine and nonpharmacological stimuli within the context of the drug self-administration paradigm in rats has generated new insights into the paradox of how nicotine, an apparently weak primary reinforcer, can sustain the robust behavior observed in self-administration and in smoking. The hypothesis presented in this paper--that nicotine acts as both a primary reinforcer and an enhancer of other nonnicotine reinforcers--provides important direction for future investigations into the neurobiology of nicotine reinforcement and treatments for smoking cessation.

Dissociating the primary reinforcing and reinforcement-enhancing effects of nicotine using a rat self-administration paradigm with concurrently available drug and environmental reinforcers

RATIONALE

Nicotine has two effects on reinforcement in traditional self-administration paradigms. It serves as a primary reinforcer by increasing the probability of behaviors that result in nicotine delivery. However, nicotine also potently enhances behaviors that result in the delivery of nonpharmacological reinforcers.

OBJECTIVES

The present study sought to dissociate these two effects of nicotine on reinforcement.

METHODS

For one group of rats (2 lever), a nonpharmacological reinforcer [visual stimulus (VS)] was available for pressing one lever. Nicotine infusions were available for pressing a different lever. A second group (NIC + VS) received more traditional self-administration training; both the VS and nicotine were delivered for pressing a single active lever. Control groups received either nicotine infusions (NIC only) or VS presentations (VS only) for pressing the active lever.

RESULTS

Nicotine alone was a weak reinforcer; the VS alone was slightly more reinforcing than nicotine. When these two reinforcers were combined (NIC + VS), response rates were synergistically increased. For the 2-lever group, responding on the nicotine lever was weak, matching the response rates of rats receiving nicotine alone. However, responding on the VS lever was potently enhanced in this group; equaling the response rates for rats receiving both reinforcers for making a single response (NIC + VS).

CONCLUSIONS

These data indicate that the reinforcement-enhancing effects of nicotine are very potent even when only moderate quantities of the drug are self-administered. Moreover, they provide the first demonstration that the reinforcement-enhancing and primary reinforcing effects of nicotine can be dissociated behaviorally.

Stimulus properties of nicotine, amphetamine, and chlordiazepoxide as positive features in a pavlovian appetitive discrimination task in rats

Recent experiments from our laboratory have demonstrated that drug states can signal when environmental cues will be followed by rewarding outcomes (ie Pavlovian conditioning). However, little is known about the generality of this approach and whether it can be used for studying the pharmacological properties of drug states. Accordingly, the present experiments tested the pharmacological specificity of nicotine (0.4 mg/kg), amphetamine (1 mg/kg), and chlordiazepoxide (CDP, 5 mg/kg) in this Pavlovian drug discrimination procedure. Following drug administration, presentation of a conditional stimulus (CS) was followed by brief access to sucrose. When saline was administered, the same CS was presented but sucrose was withheld. In substitution tests, rats in each condition received varying doses of all training drugs and caffeine. Anticipatory food seeking developed during the CS on drug sessions but not on saline sessions for all drug features (ie drug state-specific conditional response (CR)). In generalization tests, this CR decreased as a function of decreases in the training dose. Median effective doses (ED50s) were calculated for nicotine (0.054 mg/kg), amphetamine (0.26 mg/kg), and CDP (2.48 mg/kg). No compound tested substituted for the CDP training drug. Partial substitution was evident between nicotine and amphetamine; CDP did not substitute for either of these drug features. Caffeine fully substituted for nicotine (ED50 = 15.45 mg/kg) and amphetamine (ED50 = 3.70 mg/kg), but not for CDP. These results are consistent with the hypothesis that drug states can occasion appetitive Pavlovian CRs in a pharmacologically specific manner.

Nicotine serves as a feature-positive modulator of Pavlovian appetitive conditioning in rats

The present experiments examined whether a nicotine state could set the occasion for a pairing between visual cues and a rewarding outcome in rats. Following nicotine administration, presentation of a conditional stimulus (CS; light-on) was followed by brief access to a sucrose solution. When saline was administered, the same CS was presented but was not followed by any consequence. In Experiment 1, two groups assessed whether rats could acquire this Pavlovian feature-positive discrimination via different training procedures. An anticipatory food-seeking conditioned response (CR) developed during the CS on nicotine sessions but not on saline sessions in both groups. In Experiment 2, centrally acting antagonists of nicotinic acetylcholine and opiate receptors (mecamylamine and naloxone, respectively) dose-dependently blocked nicotine's control of the CR, whereas the peripherally acting nicotinic antagonist hexamethonium had no effect. Increasing or decreasing the interval between nicotine administration and testing also attenuated the CR. These results are consistent with the hypothesis that nicotine can occasion appetitive Pavlovian relations via its action at central nervous system cholinergic receptors.

Nicotine as a signal for the presence or absence of sucrose reward: a Pavlovian drug appetitive conditioning preparation in rats

RATIONALE

In Pavlovian conditioning research, nicotine is typically conceptualized as the unconditioned stimulus (US) that becomes associated with an exteroceptive conditioned stimulus (CS). This research has not explored the possibility that nicotine can also function as a CS.

OBJECTIVES

The present research examined whether nicotine served as a CS for the presence (CS+) or absence (CS-) of sucrose and started defining its specificity.

METHODS AND RESULTS

Rats trained in the CS+ condition had nicotine (0.4 mg/kg, base) paired intermittently with brief access to sucrose. Intermixed were saline sessions without sucrose. Nicotine acquired the ability to evoke goal tracking. This conditioned response (CR) decreased across extinction sessions. The CR was sensitive to nicotine dose (ED(50)=0.113 mg/kg) and administration to testing interval; 0-min and 100-min delays produced no CR. The CS properties were specific to nicotine in that amphetamine and bupropion substitution was incomplete. Rats in the CS- condition received similar discrimination training except that sucrose was paired with saline. Nicotine also served as a CS-; the saline state CS+ acquired control of goal tracking. Mecamylamine, but not hexamethonium, blocked nicotine's ability to serve as a CS+ and CS-, indicating a role for central nicotinic acetylcholine receptors.

CONCLUSIONS

Nicotine served as a signal for the presence or absence of sucrose. The extinction, CS-, and substitution results eliminated a psychomotor stimulant account. The conceptualization of nicotine as a CS suggests novel empirical research in which a drug acquires additional inhibitory and/or excitatory value based on other outcomes present during its effects.

Nicotine-conditioned locomotor sensitization in rats: assessment of the US-preexposure effect

In rats, stimulus-nicotine associations can be altered by preexposure to the nicotine US (unconditioned stimulus). This alteration differs with the conditioning preparation. In a conditioned taste avoidance preparation, preexposure to nicotine weakens conditioning. In contrast, nicotine preexposure enhances acquisition of a nicotine-conditioned place preference. No one has examined the effects of US preexposure on nicotine locomotor conditioning. In three separate experiments, we assessed the effects of nicotine preexposure on the subsequent expression of conditioned hyperactivity produced by a nicotine US. We found evidence for nicotine-conditioned locomotor sensitization in non-preexposed rats that received repeated pairings of a distinct context with the psychomotor effects of a 0.42 mg/kg dose of nicotine (free base). Conditioning was not observed at lower nicotine doses (0.18 and 0.11 mg/kg) in non-preexposed rats. Preexposure to the 0.42 and 0.18 mg/kg doses of nicotine (3 or 9 days) attenuated acute locomotor suppression and enhanced the development of locomotor sensitization to that same dose. Despite similar qualitative shifts in the locomotor profile induced by preexposure to the nicotine US, conditioned hyperactivity was only altered after 3 or 9 days of preexposure at the 0.18 mg/kg dose. Thus, similar to place conditioning, nicotine preexposure can enhance the subsequent effectiveness of the nicotine US in a locomotor conditioning preparation.

Effects of chronic caffeine pre-exposure on conditioned and unconditioned psychomotor activity induced by nicotine and amphetamine in rats

Three experiments examined the effects of chronic pre-exposure to caffeine on the subsequent conditioned and unconditioned locomotor activating effects of nicotine or amphetamine in rats. Rats were given daily intraperitoneal injections of caffeine anhydrous (0, 10 or 30 mg/kg base) for 30 days. Conditioning (environment-drug pairings) began after the last day of caffeine pre-exposure. Pre-exposure to 30 mg/kg of caffeine enhanced the acute and chronic locomotor effects of amphetamine (0.5 mg/kg). A similar enhancement of activity was not seen with the high (0.421 mg/kg base) or low dose (0.175 mg/kg) of nicotine. In a drug-free test, the distinct environment paired with amphetamine and the high dose of nicotine evoked increases in activity relative to controls. Caffeine pre-exposure did not affect expression of this conditioned hyperactivity. These effects of caffeine pre-exposure on amphetamine-induced activity could not be attributed to non-specific effects of caffeine.

Immunization to nicotine with a peptide-based vaccine composed of a conformationally biased agonist of C5a as a molecular adjuvant

This paper describes the synthesis of a nicotine hapten (Nic) that possesses a carboxyl sidearm functional group allowing for conjugation to a peptide via amide bond formation. Nic was attached to the N-terminal amino group of a 19-residue peptide composed of a conformationally biased agonist of human C5a (YSFKPMPLaR), which is used as a molecular adjuvant and a B cell epitope of human MUC1 glycoprotein (YKQGGFLGL) to yield a peptide-based nicotine vaccine, NicYKQGGFLGLYSFKPMPLaR. Rats immunized with this vaccine were significantly less sensitive to behavioral effects (a Pavlovian discrimination task) induced by their exposure to high concentrations of nicotine (0.4 mg/kg) relative to their non-vaccinated counterparts. The attenuation of these nicotine-induced behavioral effects emanated from the presence of nicotine-specific antibodies (Abs) that were present in the sera of vaccinated rats even after their repeated exposure to high concentrations of nicotine during the time required to perform the behavioral assays. These results suggest that immunization with NicYKQGGFLGLYSFKPMPLaR in the absence of adjuvant is an effective means of inducing a nicotine-specific Ab response, which is capable of attenuating nicotine-induced behavioral/psychoactive effects.

Examination of GABAergic and dopaminergic compounds in the acquisition of nicotine-conditioned hyperactivity in rats

In rats, a distinct environment repeatedly paired with nicotine (0.421 mg/kg base, s.c.) comes to evoke an increase in activity in the absence of any drug. This hyperactivity indicates a Pavlovian-conditioned association between the environment and nicotine. We investigated whether a dopamine D(1) receptor antagonist (SCH-23390), a D(2)/D(3) antagonist (eticlopride) or a GABA(B) agonist (baclofen) would prevent the acquisition of nicotine-conditioned hyperactivity. In saline-pretreated rats, acute nicotine suppressed activity during the conditioning phase (i.e. environment-nicotine pairings); chronic nicotine stimulated activity. Pretreatment with SCH-23390 (0.01 mg/kg, i.p.) attenuated the activating effects of nicotine without affecting controls. Eticlopride (0.03-0.07 mg/kg, i.p.) and baclofen (0.625 and 1.25 mg/kg, i.p.) did not affect nicotine-induced activity in a selective manner. Regardless of the pretreatment drug, rats acquired the environment-nicotine association as indexed in a drug-free test. The inability of SCH-23390 to block the acquisition of nicotine-conditioned locomotor activity is notable because in past research SCH-23390 blocked expression of the learned association.

Novel-object place conditioning: behavioral and dopaminergic processes in expression of novelty reward

In a choice situation, rats given repeated access to novel objects in one of two distinct environments display an increase in preference for the novelty-paired environment. The experiments in this present report extend the generality of this effect to new procedures. Further, this shift in preference depends on object novelty; no systematic shift in preference was observed if the environment was paired with a familiar object. Experiments in the present report also provided evidence against non-associative accounts that rely on mechanisms that leave the paired environment more novel than the unpaired environment (e.g. object interaction interfering with environmental familiarization). Consistent with a conditioning account is the loss of place conditioning when access time with the novel objects was shortened from 10 min to 5 or 2.5 min. Interestingly, although a decrease in time with objects prevented place conditioning, these groups showed a novelty-conditioned increase in activity. Finally, treatment with the dopamine D(1) antagonist SCH-23390 (0.03 mg/kg) or the dopamine D(2)/D(3) antagonist eticlopride (0.1 mg/kg) before the post-conditioning preference test blocked expression of the novel-object place conditioning. Taken together, these experiments establish that the increased preference produced by object-environment pairings reflects a conditioned association between environmental cues and the appetitive effects of receiving access to novel stimuli.

Chronic caffeine exposure in rats blocks a subsequent nicotine-conditioned taste avoidance in a one-bottle, but not a two-bottle test

Two experiments were conducted in order to investigate nicotine-conditioned taste avoidance (CTA) following chronic preexposure to caffeine. Rats were given daily intraperitoneal injections of caffeine anhydrous (0, 10, or 30 mg/kg) for 10 or 30 days. Training of the nicotine-CTA began after the last day of caffeine preexposure. On five separate occasions access to a saccharin solution was followed immediately by an injection of 1.2 mg/kg nicotine hydrogen tartrate salt or saline. Nicotine-CTA readily developed in saline-preexposed controls. That is, paired rats drank less saccharin solution than unpaired rats after repeated saccharin-nicotine pairings. A similar pattern of nicotine-CTA was found for rats preexposed to 30 mg/kg caffeine for 10 days. Following 10 days of preexposure to 10 mg/kg caffeine, however, CTA did not develop under standard testing conditions. Thirty days of caffeine preexposure did not affect the development of a nicotine-CTA even though the anorexic effects of caffeine were evident after exposure to 30 mg/kg for this duration. Thus, caffeine exposure appears to weaken acquisition or expression of the conditioned avoidance properties of nicotine. This effect is sensitive to the dose of caffeine and duration of preexposure. Importantly, the pattern of nicotine-CTA does not appear to be due to nonspecific effects of caffeine.