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

The secondary visual cortex mediated the enhancement of associative learning on methamphetamine self-administration behaviors

RATIONALE

Methamphetamine addiction is a persistent and intractable pathological learning and memory, whereas no approved therapeutics is available. However, few attentions have been paid to how associative learning participates in the formation of intractable memory related to drug addiction OBJECTIVES AND METHODS: To investigate the role of associative learning in methamphetamine addiction and the underlying neurobiological mechanism, methamphetamine self-administration, oral sucrose self-administration, chemogenetic neuromanipulation, and fiber photometry in mice were performed in this study.

RESULTS

We reported that associative learning increased methamphetamine-induced self-administration, but not oral sucrose self-administration. In addition, the enhancement of methamphetamine-induced self-administration was independent of more methamphetamine consumption, and remained with higher drug-taking and motivation in the absence of visual cues, suggesting the direct effects of the associative learning that enhanced methamphetamine-induced self-administration. Moreover, chemogenetic inactivation of the secondary visual cortex (V2) reduced the enhancement of the drug-taking induced by associative learning but did not alter sucrose-taking. Further fiber photometry of V2 neurons demonstrated that methamphetamine-associative learning elicits V2 neuron excitation, and sucrose-associative learning elicits V2 neuron inhibition.

CONCLUSIONS

Therefore, this study reveals the neurobiological mechanism of V2 excitability underlying how associative learning participates in the formation of intractable memory related to drug addiction, and gives evidence to support V2 as a promising target for stimulation therapy for methamphetamine addiction.

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.

Elucidating the reinforcing effects of nicotine: a tribute to Nadia Chaudhri

Nadia Chaudhri worked with us as a graduate student in the Center for Neuroscience at the University of Pittsburgh from 1999 until she earned her PhD in 2005, a time that coincided with the discovery in our lab of the dual reinforcing actions of nicotine, a concept that she played an important role in shaping. The research that was described in her doctoral thesis is among the foundational pillars of the now well-accepted notion that nicotine acts as both a primary reinforcer and an amplifier of other reinforcer stimuli. This reinforcement-enhancing action of nicotine is robust and likely to be a powerful driver of nicotine use. Below, we discuss the evidence that these two actions of nicotine - primary reinforcement and reinforcement enhancement - are distinct and dissociable, a finding that Nadia was closely associated with. We go on to address two other topics that greatly interested Nadia during that time, the generalizability of the reinforcement-enhancing action of nicotine to multiple classes of reinforcing stimuli and potential sex differences in the dual reinforcing actions of nicotine. The research has greatly expanded since Nadia's involvement, but the core ideas that she helped to develop remain central to the concept of the dual reinforcing actions of nicotine and its importance for understanding the drivers of nicotine use.

Nicotine but not saline self-administering or yoked control conditions produces sustained neuroadaptations in the accumbens shell

Introduction

Using yoked animals as the control when monitoring operant drug-self-administration is considered the golden standard. However, instrumental learning per se recruits several neurocircuits that may produce distinct or overlapping neuroadaptations with drugs of abuse. The aim of this project was to assess if contingent responding for nicotine or saline in the presence of a light stimulus as a conditioned reinforcer is associated with sustained neurophysiological adaptations in the nucleus accumbens shell (nAcS), a brain region repeatedly associated with reward related behaviors.

Methods

To this end, nicotine-or saline-administrating rats and yoked-saline stimulus-unpaired training conditions were assessed in operant boxes over four consecutive weeks. After four additional weeks of home cage forced abstinence and subsequent cue reinforced responding under extinction conditions, ex vivo electrophysiology was performed in the nAcS medium spiny neurons (MSNs).

Results

Whole cell recordings conducted in voltage and current-clamp mode showed that excitatory synapses in the nAcS were altered after prolonged forced abstinence from nicotine self-administration. We observed an increase in sEPSC amplitude in animals with a history of contingent nicotine SA potentially indicating higher excitability of accumbal MSNs, which was further supported by current clamp recordings. Interestingly no sustained neuroadaptations were elicited in saline exposed rats from nicotine associated visual cues compared to the yoked controls.

Conclusion

The data presented here indicate that nicotine self-administration produces sustained neuroadaptations in the nAcS while operant responding driven by nicotine visual stimuli has no long-term effects on MSNs in nAcS.

β2* nicotinic acetylcholine receptor subtypes mediate nicotine-induced enhancement of Pavlovian conditioned responding to an alcohol cue

Nicotine enhances Pavlovian conditioned responses to reward-associated cues. We investigated through which nicotinic acetylcholine receptor (nAChR) subtypes nicotine acts to produce this behavioral effect to an alcohol-associated cue. Male Long-Evans rats with freely available food and water were first accustomed to drinking 15% ethanol in their home cages using an intermittent access, two-bottle choice procedure. Then the rats were given 15 Pavlovian conditioning sessions in which a 15-s audiovisual conditioned stimulus (CS) predicted the delivery of 0.2 ml of ethanol, the unconditioned stimulus (US). Each session contained 12 CS-US trials. A control group received explicitly unpaired presentations of the CS and US. We measured Pavlovian conditioned approach to the site of US delivery during presentations of the CS, accounting for pre-CS baseline activity. Before each conditioning session, rats were injected subcutaneously with nicotine (0.4 mg/kg) or saline (1 ml/kg). During nAChR antagonist test sessions, rats were first injected systemically with the β2*-selective nAChR antagonist dihydro-beta-erythroidine (DHβE; 3 mg/kg) or the α7-selective nAChR antagonist methyllycaconitine (MLA; 6 mg/kg), followed by their assigned nicotine or saline injection before assessing their conditioned response to the alcohol-associated cue. Consistent with previous reports, nicotine enhanced the Pavlovian conditioned response to the alcohol-paired cue. DHβE attenuated this enhancement, whereas MLA did not. These results suggest that nicotine acts via β2*, but not α7, nAChRs to amplify Pavlovian conditioned responding to an alcohol cue. These findings contribute to a growing literature that identifies nAChRs as potential targets for pharmacological treatment of co-morbid alcohol and tobacco use disorders.

Neuropsychiatric Model of Addiction Simplified

While substance experimentation typically begins in adolescence, substance use disorders (SUDs) usually develop in late teens or early adulthood, often in individuals who are vulnerable because of biological and socioeconomic risk factors. Severe SUDs-synonymous with addiction-involve changes in limbic and prefrontal brain areas after chronic drug exposure. These changes involve learned associations between drug reward and cues that trigger the anticipation of that reward (known as incentive salience), as well as heightened dysphoria during withdrawal and weakened prefrontal circuits needed for inhibiting habitual responses.

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

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

Investigating sex differences and the effect of drug exposure order in the sensory reward-enhancing effects of nicotine and d-amphetamine alone and in combination

Nicotine enhances the rewarding effects of other environmental stimuli; this reward-enhancement encourages and maintains nicotine consumption. Nicotine use precedes other psychostimulant use, but receiving a stimulant prescription also predicts future smoking. Previously, no study has investigated effects of drug exposure order in reward-enhancement, nor with nicotine and d-amphetamine. Thus, we aimed to investigate how drug exposure order impacted the reward-enhancing effects of nicotine and d-amphetamine, alone and in combination. We used 20 male and 20 female Sprague-Dawley rats. Enhancement was investigated within-subjects by examining responding maintained by a visual stimulus reinforcer following a pre-session injection of either d-amphetamine (Sal, 0.1, 0.3, or 0.6 mg/kg) or nicotine (Sal, 0.03, 0.06, 0.1, 0.3 mg/kg). Twenty rats (10 M, 10 F) completed enhancement testing with nicotine before d-amphetamine. The other 20 rats (10 M, 10 F) completed testing with d-amphetamine before nicotine. Following these phases, rats were then given two pre-session injections: one of d-amphetamine (Sal, 0.1, 0.3, or 0.6 mg/kg) and another of nicotine (Sal, 0.03, 0.06, 0.1, or 0.3 mg/kg). Experiencing amphetamine before nicotine increased reward-enhancing effects of nicotine. Females exhibited greater effects of d-amphetamine on reward-enhancement, with no effect of exposure order. During the interaction phase, receiving nicotine before amphetamine enhanced the interaction between nicotine and d-amphetamine for females whereas amphetamine before nicotine heightened this interaction for males. From this, prior and current amphetamine use, in addition to sex, should be considered when treating nicotine dependency and when examining factors driving poly-substance use involving nicotine and d-amphetamine. Keywords: Adderall, ADHD, Dexedrine, operant, smoking, polysubstance use.

Lack of evidence for positive reinforcing and prosocial effects of MDMA in pair-housed male and female rats

3,4-Methylenedioxymethamphetamine (MDMA) is classified as an entactogen, producing feelings of emotional openness and relatedness. One unique feature of MDMA is that people tend to selectively take this drug in social and/or intimate situations. Although MDMA is recognized as having abuse liability, preclinical studies report that it has weak reinforcing effects in animals. The objective of this study was to characterize the positive reinforcing and prosocial effects of MDMA in a translational model of the social environment in which two rats have simultaneous and contingent access to MDMA in close physical proximity. To this end, MDMA self-administration was examined on both fixed and progressive ratio schedules of reinforcement in six groups of rats: (1) isolated males, (2) isolated females, (3) male-male dyads, (4) female-female dyads, (5) male-female dyads, and (6) female-male dyads. For pair-housed rats, data from both rats were analyzed. Next, social preferences were examined in a partner preference test. MDMA failed to produce positive reinforcing effects under all conditions examined. Across a 30-fold dose range (0.01-1.0 mg/kg/infusion), MDMA did not maintain higher responding than saline on both schedules of reinforcement and in all groups tested. In partner preference tests, a history of shared exposure to MDMA did not establish a social preference, and acute administration of MDMA failed to establish a preference for another MDMA-treated rat. These data suggest that social contact does not increase the positive reinforcing effects of MDMA in rats, and that neither contingent nor noncontingent MDMA administration establishes a social preference in rats.

Chronic nicotine increases midbrain dopamine neuron activity and biases individual strategies towards reduced exploration in mice

Long-term exposure to nicotine alters brain circuits and induces profound changes in decision-making strategies, affecting behaviors both related and unrelated to drug seeking and consumption. Using an intracranial self-stimulation reward-based foraging task, we investigated in mice the impact of chronic nicotine on midbrain dopamine neuron activity and its consequence on the trade-off between exploitation and exploration. Model-based and archetypal analysis revealed substantial inter-individual variability in decision-making strategies, with mice passively exposed to nicotine shifting toward a more exploitative profile compared to non-exposed animals. We then mimicked the effect of chronic nicotine on the tonic activity of dopamine neurons using optogenetics, and found that photo-stimulated mice adopted a behavioral phenotype similar to that of mice exposed to chronic nicotine. Our results reveal a key role of tonic midbrain dopamine in the exploration/exploitation trade-off and highlight a potential mechanism by which nicotine affects the exploration/exploitation balance and decision-making.

Reward-enhancing effects of d-amphetamine and its interactions with nicotine were greater in female rats and persisted across schedules of reinforcement

Nicotine enhances the value of environmental stimuli and rewards, and reward enhancement can maintain nicotine consumption. Stimulants such as d-amphetamine are misused more by women and are commonly co-used with nicotine. d-Amphetamine potentiates nicotine's effects in human and animal research. To date, there are no published studies examining this interaction in a reward-enhancement task. The current study sought to investigate the reward-enhancing effects of nicotine alongside and coadministered with d-amphetamine. Further, we evaluated the persistence of reward enhancement across ratio and temporal schedules of reinforcement. We used 10 male and 10 female Sprague-Dawley rats. Enhancement was assessed within subjects by examining active lever pressing for a visual stimulus reinforcer on variable ratio 3, variable interval 30 s and variable time 30 s - variable ratio 3 schedules. Before 1-h sessions, rats received one injection of saline, 0.1 or 0.3 mg/kg d-amphetamine and one of saline or 0.4 mg/kg nicotine, making six possible drug combinations (saline + saline, saline + nicotine, 0.1 d-amphetamine + aline, 0.1 d-amphetamine + nicotine, 0.3 d-amphetamine + saline and 0.3 d-amphetamine + nicotine) experienced in a randomized order by each rat. When d-amphetamine was coadministered with nicotine, we found an interaction effect on reward enhancement that persisted across schedules of reinforcement. Males and females exhibited reward enhancement by 0.3 d-amphetamine, while only females showed reward enhancement by 0.1 d-amphetamine. Further, females responded more for the visual stimulus than males in all d-amphetamine conditions. Future studies should assess how reward enhancement is involved in high nicotine-amphetamine comorbidity rates and enhanced amphetamine misuse in women.

Conditioned enhancement of the nicotine reinforcer

This study was designed to assess whether nicotine can acquire additional reinforcing properties through associations with other rewards. To this end, rats self-administered nicotine-alone (0.01 mg/kg) or nicotine paired with access to sucrose during the conditioning phase. In the subsequent challenge phase, we tested the effect of nicotine-sucrose pairings on the reinforcing effects of nicotine using a progressive ratio schedule of reinforcement. Using this approach, we show that (a) rats in both paired and nicotine-alone conditions self-administered similar amounts of nicotine in the initial conditioning phase of the study when intake was limited to 10 infusions per session, (b) nicotine rapidly acquired control over goal-tracking behavior in the paired condition, (c) rats that had a history of nicotine and sucrose pairings worked harder and took more nicotine as measured on a progressive ratio using a distinct response form, and (d) conditioned goal-tracking evoked by nicotine did not show extinction when sucrose was no longer paired with nicotine over the 11 days of nicotine self-administration on a progressive ratio schedule of reinforcement. Overall, our results demonstrate that in addition to the multifaceted nature of nicotine stimulus that includes primary reinforcing effects, conditioned reinforcing effects, and reward enhancing effects, nicotine can also acquire additional reinforcing properties through associations with other rewards. This ability to acquire additional reinforcing properties through associative learning may contribute to the development and perpetuation of tobacco use disorder. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

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

RATIONALE AND OBJECTIVES

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Female rats display higher methamphetamine-primed reinstatement and c-Fos immunoreactivity than male rats

Methamphetamine (meth) dependence is often characterized by persistent and chronic relapse (i.e., return to drug use). Previous work suggests females may be at greater risk to relapse. In this study, we extended this limited evidence and identified sex-dependent neural substrates related to meth-triggered reinstatement. Male and female Sprague-Dawley rats were implanted with indwelling jugular catheters. Half of the rats were then trained to self-administer meth (0.05 mg/kg/inf); the other half self-administered saline during 21 daily sessions (2 h). Rats were then given 12 extinction sessions. Twenty-four hours after the last extinction session, rats received reinstatement testing. Half of the rats received a meth-prime (0.3 mg/kg, IP) injection and the remaining rats received a saline injection. This design resulted in 4 separate groups for each sex, allowing for careful investigation of brain regions related to meth-triggered reinstatement. Brains were harvested following the reinstatement session and c-Fos immunoreactivity was measured in multiple brain regions. Meth triggered reinstatement in both sexes and this effect was more robust in females compared to males. Significant sex differences were detected. Females showed greater c-Fos immunoreactivity in the cingulate cortex area 1, lateral orbitofrontal cortex, prelimbic cortex, caudate-putamen, nucleus accumbens core and shell, and central nucleus of the amygdala following meth-primed reinstatement.

Polysubstance use in the U.S. opioid crisis

Interventions to address the U.S. opioid crisis primarily target opioid use, misuse, and addiction, but because the opioid crisis includes multiple substances, the opioid specificity of interventions may limit their ability to address the broader problem of polysubstance use. Overlap of opioids with other substances ranges from shifts among the substances used across the lifespan to simultaneous co-use of substances that span similar and disparate pharmacological categories. Evidence suggests that nonmedical opioid users quite commonly use other drugs, and this polysubstance use contributes to increasing morbidity and mortality. Reasons for adding other substances to opioids include enhancement of the high (additive or synergistic reward), compensation for undesired effects of one drug by taking another, compensation for negative internal states, or a common predisposition that is related to all substance consumption. But consumption of multiple substances may itself have unique effects. To achieve the maximum benefit, addressing the overlap of opioids with multiple other substances is needed across the spectrum of prevention and treatment interventions, overdose reversal, public health surveillance, and research. By addressing the multiple patterns of consumption and the reasons that people mix opioids with other substances, interventions and research may be enhanced.

High-dose adolescent nicotine exposure permits spontaneous nicotine self-administration in adult male rats

INTRODUCTION

While cigarette smoking rates have been steadily decreasing over the past decade, there has been a dramatic increase in nicotine use via e-cigarettes, especially during adolescence. Adolescent e-cigarette use is associated with a greater risk of future cigarette smoking, and increased rates of cigarette smoking in individuals who may have otherwise never tried cigarettes. In humans and rodents, early initiation of nicotine use has been associated with greater consumption, dependence, and persistent nicotine use. The present study sought to investigate the long-lasting effect of daily high-dose nicotine exposure during adolescence on nicotine consumption in adulthood.

METHOD

Male Sprague-Dawley rats were exposed daily to nicotine (1.0 mg/kg, subcutaneous), or vehicle (1 mL/kg saline, subcutaneous) during adolescence (post-natal day [P] 28-41). Adult nicotine self-administration (0.02 mg/kg/infusion, intravenous) was assessed beginning on P75 on fixed-ratio 1 (FR1), fixed-interval 1 min (FI1), and progressive ratio (PR) schedules of reinforcement.

RESULTS

Adolescent nicotine pre-exposure did not affect adult nicotine self-administration on the simple FR1 schedule, however increased intake and responding for nicotine was observed when a short delay was implemented on an FI1 schedule of reinforcement.

CONCLUSIONS

Adolescence is a critical period when the brain is especially vulnerable to the effects of nicotine. Nicotine exposure in adolescence enhances susceptibility to increased nicotine intake in adulthood on a reinforcement schedule more reflective of human nicotine intake patterns, and this effect can extend into adulthood even after termination of nicotine exposure during adolescence.

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.

Sex Differences in the Reward-Enhancing Effects of Nicotine on Ethanol Reinforcement: A Reinforcer Demand Analysis

BACKGROUND

Alcohol is often consumed with tobacco, and dependence to alcohol and tobacco are highly comorbid. In addition, there are differences in the prevalence of nicotine- and alcohol-abuse between the sexes. Nicotine produces enhancing effects on the value of other reinforcers, which may extend to alcohol.

METHODS

Male and female Wistar rats were trained to self-administer 15% ethanol solution in 30-minute sessions. Once ethanol self-administration was established, demand for ethanol was evaluated using an exponential reinforcer demand method, in which the response cost per reinforcer delivery was systematically increased over blocks of several sessions. Within each cost condition, rats were preinjected with nicotine (0.05, 0.1, 0.2, or 0.4 mg/kg base, SC) or saline 5 minutes before self-administration sessions. The effects of nicotine dose and biological sex were evaluated using the estimates generated by the reinforcer demand model.

RESULTS

Under saline conditions, males showed greater sensitivity to ethanol reinforcement than females. Nicotine enhanced the reinforcement value of alcohol and this varied with sex. In both sexes, 0.4 mg/kg nicotine decreased intensity of ethanol demand. However, 0.05, 0.1, and 0.2 mg/kg nicotine decreased elasticity of ethanol demand in females, but not in males.

CONCLUSIONS

Nicotine enhances ethanol reinforcement, which may partially drive comorbidity between nicotine-abuse and alcohol-abuse. Males showed signs of greater ethanol reinforcement value than females under saline conditions, and nicotine attenuated this effect by increasing ethanol reinforcement value in the females. These findings highlight that a complete understanding of alcohol-abuse must include a thorough study of alcohol use in the context of other drug use, including nicotine.

IMPLICATIONS

Nicotine dose dependently enhances the alcohol reinforcement value in a manner that is clearly influenced by biological sex. Under saline baseline conditions, males show lower elasticity of demand for alcohol reinforcement than females, indicative of greater reinforcement value. However, nicotine attenuated this difference by enhancing alcohol reward in the females. Specifically, low-to-moderate doses (0.05-0.2 mg/kg) of nicotine decreased elasticity of alcohol demand in female rats, increasing the perseverance of their alcohol taking behavior. These data indicate that the well-documented reward-enhancing effects of nicotine on sensory reinforcement extend to alcohol reinforcement and that these vary with biological sex.

Nicotine Facilitation of Conditioned Place Preference to Food Reward in Humans

Nicotine has recently been shown to enhance the motivational value of non-nicotine stimuli in nonhumans. To investigate whether nicotine also enhances reward in humans, we used a virtual translation of the conditioned place preference (CPP) paradigm to examine nicotine's reward-enhancing effects using a low-dose 2 mg nicotine lozenge targeted to a mild use population. Methods: Sixty-eight nicotine-using undergraduates were randomly assigned to receive either a 2 mg nicotine or placebo lozenge prior to conditioning. During each of six, three-minute conditioning sessions, participants were confined to one of two VR rooms. In one room, they received real chocolate M&Ms, whereas no M&Ms were administered in the other room. Following conditioning, a three-minute free-access test session occurred during which participants had unrestricted access to both rooms without reward. Results: Individuals who received nicotine demonstrated a CPP by spending significantly more time in the room previously paired with M&Ms compared to the unrewarded room (p = 0.04). Those who received placebo did not demonstrate a CPP (p > 0.05). Moreover, we observed no significant differences between treatment groups in terms of the amount of time spent in each virtual room. Conclusion: While nicotine seems to facilitate CPP expression for a virtual environment previously paired with chocolate food rewards, further characterization of the mechanism by which this occurs is needed.

A Conceptual Framework for Understanding Multiple Tobacco Product Use and the Impact of Regulatory Action

INTRODUCTION

Over 35% of the adult tobacco-using population regularly use more than one tobacco product. Although rates of tobacco use in the United States have declined over the last decade, rates of multiple tobacco product (MTP) have either remained stable (among adults) or increased (among youth).

METHODS

In this paper, we review the literature and propose a framework for understanding both MTP use and how regulatory actions on any single tobacco product (STP) may influence the use of other tobacco products.

RESULTS AND CONCLUSIONS

Within the framework, Product, Person, and Context/Situational factors (and their interactions) influence product cross-substitution and thus patterns of use of MTPs. In addition, we propose that Context/Situation effects specifically increase the complexity of MTP-use patterns resulting in "dynamic complementarity" in addition to substitution-like relationships between tobacco products. Experimentation with, and use of, various tobacco products results in reinforcement histories that affect which products are used, in what contexts, and by whom, which in turn has downstream impacts on toxicant exposure and health. We conclude our analysis with an examination of how regulation of STPs can have impacts on the use of other STP and MTP use and provide research questions for further examining MTP use.

IMPLICATIONS

Though rates of tobacco use have declined in the United States, over 35% of the adult tobacco-using population regularly uses more than one tobacco product. This paper provides a framework for understanding MTP use and how regulatory actions on any STP may influence the use of other tobacco products. We conclude our analysis by providing research questions for further examining MTP use.

Detrimental Effects of a Retrieval-Extinction Procedure on Nicotine Seeking, but Not Cocaine Seeking

Retrieval-extinction memory reactivation procedures have been used to prevent the return of learned fear and drug seeking in preclinical models. These procedures first reactivate the original memory with a brief cue exposure (i.e., retrieval) session, and then disrupt memory reconsolidation by conducting extinction training within the reconsolidation window. The original memory is thought to be updated with the new information conveyed by extinction learning, resulting in a persistent therapeutic effect beyond that observed with extinction training alone (i.e., no retrieval). Here, we attempted to replicate the therapeutic effects on cocaine seeking reported by Xue et al. (2012), and extend these findings to nicotine seeking. Rats self-administered either cocaine or nicotine with contingent cues for weeks, and were then divided into two groups. The retrieval group underwent a 10-min retrieval session wherein drug cues were available, but drug was not. Ten minutes later, they were allowed to continue cue extinction training for an additional 60 min. The no retrieval group underwent a contiguous 70-min cue extinction session. These procedures continued for weeks, followed by a test for spontaneous recovery of drug seeking. No group differences were observed on any measure of cocaine seeking, although both groups exhibited extinction and spontaneous recovery. By contrast, for nicotine seeking, the retrieval group exhibited resistance to extinction, an effect that persisted on the spontaneous recovery test. These findings underscore the importance of drug type in the outcome of retrieval-extinction procedures and moreover indicate that retrieval-extinction procedures can be detrimental to nicotine seeking.

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.

Optimization of a nicotine degrading enzyme for potential use in treatment of nicotine addiction

BACKGROUND

Smoking and tobacco use continue to be the largest preventable causes of death globally. A novel therapeutic approach has recently been proposed: administration of an enzyme that degrades nicotine, the main addictive component of tobacco, minimizing brain exposure and reducing its reinforcing effects. Pre-clinical proof of concept has been previously established through dosing the amine oxidase NicA2 from Pseudomonas putida in rat nicotine self-administration models of addiction.

RESULTS

This paper describes efforts towards optimizing NicA2 for potential therapeutic use: enhancing potency, improving its pharmacokinetic profile, and attenuating immunogenicity. Libraries randomizing residues located in all 22 active site positions of NicA2 were screened. 58 single mutations with 2- to 19-fold enhanced catalytic activity compared to wt at 10 μM nicotine were identified. A novel nicotine biosensor assay allowed efficient screening of the many primary hits for activity at nicotine concentrations typically found in smokers. 10 mutants with improved activity in rat serum at or below 250 nM were identified. These catalytic improvements translated to increased potency in vivo in the form of further lowering of nicotine blood levels and nicotine accumulation in the brains of Sprague-Dawley rats. Examination of the X-ray crystal structure suggests that these mutants may accelerate the rate limiting re-oxidation of the flavin adenine dinucleotide cofactor by enhancing molecular oxygen's access. PEGylation of NicA2 led to prolonged serum half-life and lowered immunogenicity observed in a human HLA DR4 transgenic mouse model, without impacting nicotine degrading activity.

CONCLUSIONS

Systematic mutational analysis of the active site of the nicotine-degrading enzyme NicA2 has yielded 10 variants that increase the catalytic activity and its effects on nicotine distribution in vivo at nicotine plasma concentrations found in smokers. In addition, PEGylation substantially increases circulating half-life and reduces the enzyme's immunogenic potential. Taken together, these results provide a viable path towards generation of a drug candidate suitable for human therapeutic use in treating nicotine addiction.

An extended history of drug self-administration results in multiple sources of control over drug seeking behavior

It is widely recognized that across the development of drug addiction, cues associated with drug use come to exert increasing control over drug seeking and taking behaviors. However, there remain gaps in our knowledge regarding how the different types of drug related cues affect drug seeking and taking behaviors, and how the emergence of cue control over these behaviors relates to the onset of drug seeking compulsions. This paper reviews the literature on drug self-administration in animals to address these gaps. It first identifies the different types of cues that acquire control over reward seeking behavior generally, and examines whether the same types of cues acquire control over drug seeking behavior specifically. It then examines how the role of drug related cues in motivating and reinforcing drug seeking behavior changes across an extended drug-taking history, with a particular focus on the case of nicotine. The evidence reviewed shows that, after an extended history of drug taking, drug seeking behaviors are controlled by contextual cues associated with the development of drug seeking habits, response contingent cues that accompany delivery of the drug, as well as internal states that correlate with levels of drug intake. These multiple sources of control over drug seeking are discussed in relation to the generation of an addicted phenotype in animal models and the hypothesized progression from internal control over drug use to compulsive drug seeking.

Defining the place of habit in substance use disorders

It has long been suggested that alcohol or substance use disorders could emerge from the progressive development and dominance of drug habits. Like habits, drug-related behaviors are often triggered by drug-associated cues. Like habits, addictive behaviors are strong, rigid and "hard to break". Like habits, these behaviors are insensitive to their outcome and persist despite negative consequences. "Pathological habit" thus appears as a good candidate to explain the transition to compulsive drug use. However, drug use could also be considered as a goal-directed choice, driven by the expectation of drug outcomes. For example, drug addicts may engage in drug-seeking behaviors because they view the drug as more valuable than available alternatives. Substance use disorders therefore may not be all about habit, nor fully intentional, and could be considered as resulting from an imbalance between goal-directed and habitual control. The main objective of this review is to disentangle the relative contribution of habit formation and impairment of goal-directed behavior in this unbalanced control of addictive behaviors. Although deficits in goal-directed behavior have been demonstrated in alcohol and substance use disorders, reliable demonstration of abnormal habit formation has been curtailed by the paucity of paradigms designed to assess habit as a positive result. Refining our animal and human model of habit is therefore required to precisely define the place of habit in substance use disorders and develop appropriate and adapted neurobehavioral treatments.

The effect of nicotine and nicotine+monoamine oxidase inhibitor on the value of alcohol

Alcohol is the most commonly abused drug in the USA and many people suffer from alcohol use disorder. Many factors are associated with alcohol use disorder, but the causal role of comorbid nicotine use has not been extensively considered. Nicotine has reward-enhancing properties and may increase the value of alcohol. Monoamine oxidase inhibition increases nicotine self-administration and may increase the reward-enhancing effects of nicotine. We assessed the effect of nicotine and nicotine in combination with a commonly used monoamine oxidase inhibitor (tranylcypromine) on the value of alcohol using a progressive ratio schedule of reinforcement in rats. Nicotine administration increased the breakpoint for alcohol, but nicotine in combination with tranylcypromine decreased the breakpoint for alcohol. The current study adds to previous research showing that nicotine increases the value of alcohol. This finding has important implications for the etiology of addiction because of the comorbidity of smoking with many drugs of abuse. The finding that nicotine in combination with tranylcypromine reduces the value of alcohol warrants further investigation.

Nicotine effects on associative learning in human non-smokers

Tobacco smoking is the most common preventable cause of death in the US. Nicotine is considered the primary constituent responsible for tobacco addiction. Its paradoxically high abuse potential may reflect behavioral control by drug-associated stimuli, which appears to play a larger role for tobacco dependence than for other abused drugs. We tested a potential explanation, hypothesizing that nicotine enhances associative learning, the mechanism underlying the conditioning of drug-associated stimuli. Thirty-two non-smokers were exposed to transdermal nicotine (7 mg/24 h) and placebo in a double-blind cross-over study and tested with behavioral paradigms designed to isolate incidental stimulus-stimulus or stimulus-response learning. The stop signal task required speeded gender judgments of face stimuli. A tone signaled when to withhold the response. Unbeknownst to participants, some faces were always paired with stop trials. Nicotine enhanced the facilitation of stop-responses to these stimuli, and the slowing of go-responses when previously stop-associated stimuli were paired with go trials, indicating stronger associations between paired stimuli and the stop signal/stop response. Another task required feedback-based learning of associations between pairs of shape stimuli. Five pairs were made from either ten different stimuli, or from different combinations of two identical sets of five stimuli with correct associations depending on contextual information. Nicotine increased incorrect choices of stimuli that were associated in a different context, indicating stronger stimulus-stimulus associations at the expense of flexible context-adaptive behavior. The results indicate that nicotine can enhance incidental associative learning, a mechanism that may promote the formation of smoking-associated stimuli and cue-controlled drug-taking.

Social reinforcement as alternative to sucrose reinforcement is increased by nicotine and methylphenidate in male Fischer-344 rats

BACKGROUND

Stimulant drugs such as nicotine (NIC) and methylphenidate (MPH) are hypothesized to increase the reinforcing value of sensory stimuli, thus increasing the effectiveness of such reinforcers as alternatives to sucrose reinforcers.

METHODS

Inbred Fischer-344 rats (n = 30) were assigned to three groups: saline (SAL; n = 10), nicotine (NIC; n = 10), or methylphenidate (MPH; n = 10). Testing was done in three phases: sucrose only, (SUC), sucrose and drug (SUC/DRUG), and sucrose, drug, and social reinforcement (SUC/DRUG/SOC). During the SUC phase, rats were trained on a progressive ratio 5 (PR5) reinforcement schedule for sucrose (20% solution). In the SUC/DRUG phase, animals were treated with SAL, NIC (0.4 mg/kg, n = 10 SC), or MPH (2.0 mg/kg, n = 10 IP) 30 min prior to testing. In the SUC/DRUG/SOC phase, animals continued receiving drug treatment, and social reinforcement was introduced concurrently with the sucrose reinforcer. The progressive ratio for each reinforcer ran independently of the others. Reinforcing value was measured as break point (BP), the highest number of responses resulting in a reinforcer.

RESULTS

SAL-treated animals showed no significant change in sucrose BP. MPH-treated animals showed decreased sucrose BP in the SUC/DRUG phase, with a further reduction in the SUC/DRUG/SOC phase. NIC-treated animals decreased sucrose BP only when a social alternative was offered.

CONCLUSION

Both NIC and MPH reduce the sucrose BP in the presence of a social alternative. The decrease in sucrose responding, coupled with increased social responding, suggests that the social alternative acted as an effective alternative reinforcer to sucrose. From a translational perspective, these results suggest that stimulant drugs such as NIC and MPH may increase the effectiveness of treatments that use alternative social reinforcers to decrease eating.

Nicotine drug discrimination and nicotinic acetylcholine receptors in differentially reared rats

RATIONALE

Individuals vary in sensitivity to the behavioral effects of nicotine, resulting in differences in vulnerability to nicotine addiction. The role of rearing environment in determining individual sensitivity to nicotine is unclear. The neuropharmacological mechanisms mediating the effect of rearing environment on the behavioral actions of nicotine are also poorly understood.

OBJECTIVES

The contribution of rearing environment in determining the sensitivity to the interoceptive effects of nicotine was determined in rats reared in isolated conditions (IC) or enriched conditions (EC). The role of dopamine receptors and α4β2*-nicotinic acetylcholine (nACh) receptors in mediating the differential effect of IC and EC on the interoceptive action of nicotine was determined.

METHODS

The interoceptive action of nicotine was measured as the discriminative stimulus effect of nicotine. Mecamylamine- and eticlopride-inhibition of the nicotine stimulus were used to examine nACh and dopamine receptors, respectively. α4β2*-nACh receptor expression in the mesolimbic dopamine pathway was determined by quantitative autoradiography of [¹²⁵I]-epibatidine binding.

RESULTS

EC-reared rats are less sensitive than IC-reared rats to the discriminative stimulus effects of nicotine at all but maximally effective doses. Mecamylamine inhibited the nicotine stimulus threefold more potently in EC-reared rats (IC₅₀ = 0.25 mg/kg) compared to IC-reared rats (IC₅₀ = 0.75 mg/kg); eticlopride inhibition was not different. [¹²⁵I]-epibatidine binding in the ventral tegmental area of EC-reared rats was reduced (2.8 ± 0.3 fmol) compared to that of IC-reared rats (4.0 ± 0.4 fmol); there was no difference in the nucleus accumbens.

CONCLUSIONS

Rearing environment regulates the sensitivity to the interoceptive effects of nicotine and α4β2*-nACh receptor expression in the mesolimbic dopamine pathway.

Effects of nicotine self-administration on incentive salience in male Sprague Dawley rats

RATIONALE

Prolonged use of nicotine appears to enhance incentive salience, a motivational-cognitive process that transforms an otherwise neutral stimulus into a "wanted" stimulus. It has been suggested that nicotinic enhancement of incentive salience contributes to the potential of relapse in individuals with tobacco addiction. However, there are two main limitations of prior research that caution this claim: (a) the use of passive experimentally delivered nicotine and (b) the use of sign-tracking as an index of incentive salience, without acknowledging the competing nature of goal- and sign-tracking responses.

OBJECTIVES

To determine whether nicotinic enhancement of incentive salience attributed to non-nicotinic stimuli occurs when rats self-administer nicotine, and whether it is facilitated by a prior history of nicotine self-administration.

METHODS

Twenty-three male rats were trained daily, for 24 days, on a nicotine self-administration (SA) paradigm in the morning, and on a four-conditioned-stimuli Pavlovian conditioned approach (4-CS PCA) task in the afternoon. Self-administration was followed by extinction and cue reinstatement. A subcutaneous nicotine challenge was performed during the last 7 days of the study.

RESULTS

Nicotine self-administration selectively enhanced sign-tracking in the 4-CS PCA. Upon extinction, sign-tracking quickly declined to control levels. Experimenter-administered nicotine enhanced sign-tracking similarly regardless of nicotine history.

CONCLUSIONS

The results suggest that nicotinic enhancement of incentive salience is transient, and a previous history of nicotine use does not cause further sensitization. Taken together, these results suggest that nicotine enhances incentive salience, particularly-and perhaps exclusively-while onboard.

Reinforcement enhancement by nicotine in adult rats: behavioral selectivity and relation to mode of delivery and blood nicotine levels

RATIONALE

Reinforcement-enhancing effects of nicotine occur in human subjects and laboratory rats. However, the doses used in animal studies typically exceed smoking-associated levels of exposure, and generalized behavioral activation by nicotine can potentially confound data interpretation.

METHODS

During daily 60-min sessions, male adult rats pressed an "active" lever to illuminate a brief cue light. Pressing on either the active or inactive lever retracted both levers for 60 s. Nicotine (0.025-0.2 mg/kg) was given either by continuous intravenous (IV) infusion, or spaced IV pulses (3-s or 30-s/pulse), or pre-session subcutaneous (SC) injection.

RESULTS

Almost all rats responded preferentially for the cue light for several weeks. After several home-cage nicotine injections, reinforcement enhancement occurred even within the first nicotine test session. Nicotine increased active lever responding without altering inactive lever responding, with effects reliably observed at doses as low as 0.1 mg/kg SC or 0.1 mg/kg/session IV. Within the session, the 0.1 mg/kg dose maximally increased active lever responding by 2-3-fold, coinciding with serum levels of 25 ng/ml. Intravenous nicotine (tested at 0.1 mg/kg/60-min session) was equally effective whether delivered by continuous infusion or in a series of equally spaced 0.003 mg/kg pulses each of 3-s or 30-s duration.

CONCLUSIONS

Low doses of nicotine can potentiate responding for a primary sensory reinforcer without producing a generalized increase in lever pressing. Reinforcer enhancement by nicotine generalized to several modes of drug delivery, appeared to track circulating levels of drug, and occurred even at serum levels within the daytime range of moderate cigarette smokers.

Nicotine-enhanced Pavlovian conditioned approach is resistant to omission of expected outcome

Conditioned stimuli contribute to the resilience of nicotine addiction in that nicotine-associated cues can influence smokers and promote relapse. These stimuli are thought to acquire incentive motivational properties through a Pavlovian mechanism, and this phenomenon can be measured in animals by observing conditioned approach to the conditioned stimulus (sign-tracking) or to the location of unconditioned stimulus delivery (goal-tracking). Goal-tracking is thought to be more flexible than sign-tracking in response to changes in expected outcome. Nicotine exposure can increase the expression of conditioned responses, and we hypothesized that animals exposed to nicotine would also exhibit less flexible conditioned responses after a change in the expected unconditioned stimulus. Adult male rats were exposed to nicotine (0.4mg/kg, s.c.) or saline before Pavlovian conditioned approach training sessions. After training, animals underwent test sessions that reduced (water substitution) or withheld (omission) the unconditioned stimulus (US, 20% sucrose). As expected, nicotine enhanced sign- and goal-tracking. Water substitution moderately and nonspecifically reduced both sign- and goal-tracking in all rats. In contrast, US omission only reduced goal-tracking, with robust effects in saline-exposed rats and smaller effects in nicotine-exposed rats. These data support the hypothesis that both sign-tracking and nicotine exposure confer behavioral inflexibility under US omission.

The effects of nicotine on conditioning, extinction, and reinstatement in humans

Nicotine has been shown to enhance the reinforcement and reward-responsiveness of non-nicotine stimuli. To determine whether nicotine enhances the strength of conditioning to context, undergraduate participants with varying levels of nicotine dependence were recruited for a two-day study and tested on a virtual reality (VR) conditioned place preference (CPP) paradigm. On day one, participants explored two virtual rooms where they received multiple pairings of M&M rewards in one room and no rewards in the other room, followed by a free-access test session with no rewards. On day two, participants received three test sessions to assess extinction. Subsequently, participants received M&Ms. in a novel context and were then tested for reinstatement. Prior to testing on each day, subjects were administered either nicotine (4mg) or placebo lozenges, in a between-subjects, four-group, 2×2 design (nicotine or placebo on days 1 and 2). After conditioning on day one, only participants who received placebo exhibited a CPP by spending significantly more time in the room previously-paired with M&Ms. Contrary to our hypothesis, nicotine-treated participants did not display a significant CPP, and there were no significant differences between treatment groups. However, post hoc analysis indicated that in a subset of participants with greater nicotine dependence, the nicotine group displayed a CPP by rating the M&M-paired room as significantly more enjoyable than those who received placebo. Additionally, while neither treatment group showed significant place preferences during the first two extinction sessions on Day 2, individuals who received nicotine on Day 1 or placebo on Day 2 spent significantly more time in the M&M-paired room during the final extinction session. Finally, those who received nicotine on Day 2 exhibited significantly greater reinstatement compared to placebo-treated participants. These results partially support preclinical evidence that nicotine can affect learning, extinction, and reinstatement.

Effects of the phencyclidine model of schizophrenia and nicotine on total and categorized ultrasonic vocalizations in rats

Patients with schizophrenia smoke cigarettes at a higher rate than the general population. We hypothesized that a factor in this comorbidity is sensitivity to the reinforcing and reinforcement-enhancement effects of nicotine. Phencyclidine (PCP) was used to model behavioral changes resembling negative symptoms of schizophrenia in rats. Ultrasonic vocalizations (USVs) in rats have been used to measure emotional states, with 50 kHz USVs indicating positive states and 22 kHz USVs indicating negative states. Total and categorized numbers of 22 and 50 kHz USVs and USVs during a visual stimulus (e.g. a potential measure of reinforcement-enhancement) were examined in rats following injection of PCP (2.0 mg/kg) and/or nicotine (0.2 or 0.4 mg/kg) daily for 7 days. PCP was then discontinued and all rats received nicotine (0.2 and 0.4 mg/kg) and PCP (2.0 mg/kg) on three challenge days. PCP acutely decreased 50 kHz vocalizations, whereas repeated nicotine potentiated rates of vocalizations, with similar patterns during light presentations. Rats in the PCP and nicotine combination groups made more 50 kHz vocalizations compared with rats in the control groups on challenge days. We conclude that PCP may produce a reward deficit, which is shown by decreased 50 kHz USVs, and behaviors post-PCP exposure may best model the comorbidity between schizophrenia and nicotine.

Intravenous self-administration of alcohol in rats-problems with translation to humans

Alcohol is consumed orally by humans, and oral self-administration has been successfully modeled in laboratory animals. Over the last several years, attempts have been made to develop a procedure for the reliable intravenous (IV) self-administration of alcohol in rodents. IV self-administration would provide a better tool for investigating neurobiological mechanisms of alcohol reinforcement and dependence because confounding factors associated with oral self-administration, such as variations in orosensory sensitivity to alcohol and/or its absorption, are avoided. A review of the literature shows that rats, mice and non-human primates can initiate and maintain IV self-administration of alcohol. However, there are 50- to 100-fold interspecies differences in the reported alcohol infusion doses required. Most surprising is that the infusion dose (1-2 mg/kg) that reliably maintains IV alcohol self-administration in rats results in total alcohol intakes of only 20-25 mg/kg/hour, which are unlikely to have significant pharmacological effects. The evidence to support IV self-administration of such low doses of alcohol in rats as well as the potential biological mechanisms underlying such self-administration are discussed. The minute amounts of alcohol shown to reliably maintain IV self-administration behavior in rats challenge the relationship between their blood alcohol levels and the rewarding and reinforcing effects of alcohol.

Effects of environmental enrichment on d-amphetamine self-administration following nicotine exposure

Adolescent nicotine exposure has been shown to lead to further psychostimulant use in adulthood. Previous preclinical research in rats has shown that environmental enrichment may protect against drug abuse vulnerability. The current study was designed to examine whether environmental enrichment can block the ability of adolescent nicotine exposure to increase d-amphetamine self-administration in adulthood. Male Sprague-Dawley rats were raised in either enriched conditions (ECs) or isolated conditions (ICs) and then injected with saline or nicotine (0.4 mg/kg, sc) for 7 days during adolescence. In adulthood rats were allowed to self-administer d-amphetamine under a fixed ratio (FR; 0, 0.006, 0.01, 0.02, 0.06, and 0.1 mg/kg/infusion) and progressive ratio (PR; 0, 0.006, 0.06, and 0.1 mg/kg/infusion) schedule of reinforcement. Nicotine-treated IC rats self-administered more d-amphetamine at 0.006, 0.01, and 0.02 mg/kg/infusion doses compared with their saline-treated IC counterparts regardless of the schedule maintaining behavior. This effect of nicotine was reversed in EC rats on a fixed ratio schedule. These findings indicate that environmental enrichment can limit the ability of adolescent nicotine exposure to increase vulnerability to other psychostimulant drugs, such as d-amphetamine. (PsycINFO Database Record

Nicotine Acutely Enhances Reinforcement from Non-Drug Rewards in Humans

Preclinical research documents that, aside from the primary and secondary reinforcing effects of nicotine intake itself, nicotine also acutely enhances the reinforcing efficacy of non-drug reinforcers ("rewards"). Study of these effects in humans has largely been overlooked, but very recent findings suggest they may have clinical implications for more fully understanding the persistence of tobacco dependence. This overview first outlines the topic and notes some recent human studies indirectly addressing nicotine effects on related responses (e.g., subjective ratings), explaining why those findings do not directly confirm enhancement of behavioral reinforcement per se due to nicotine. Then, the methodology used in the subsequently presented studies is described, demonstrating how those studies specifically did demonstrate enhancement of reinforced responding for non-drug rewards. The main section focuses on the limited controlled research to date directly assessing nicotine's acute reinforcement-enhancing effects in humans, particularly as it relates to reinforced behavioral responding for non-drug rewards in non-human animal models. After detailing those few existing human studies, we address potential consequences of these effects for dependence and tobacco cessation efforts and then suggest directions for future research. This research indicates that nicotine per se increases responding in humans that is reinforced by some rewards (auditory stimuli via music, visual stimuli via video), but perhaps not by others (e.g., money). These reinforcement-enhancing effects in smokers are not due to dependence or withdrawal relief and can be restored by a small amount of nicotine (similar to a smoking lapse), including from e-cigarettes, a non-tobacco nicotine product. Future clinical research should examine factors determining which types of rewards are (or are not) enhanced by nicotine, consequences of the loss of these nicotine effects after quitting smoking, potential individual differences in these effects, and the possibility that nicotine via nicotine replacement therapy and non-nicotine quit medications may attenuate loss of these effects upon quitting. Further study with humans of nicotine's reinforcement-enhancing effects may provide a more complete understanding of smoking persistence and added mechanisms of cessation medication efficacy.

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.

Effects of Monoamine Oxidase Inhibition on the Reinforcing Properties of Low-Dose Nicotine

The Food and Drug Administration (FDA) has the authority to regulate cigarette smoke constituents, and a reduction in nicotine content might benefit public health by reducing the prevalence of smoking. Research suggests that cigarette smoke constituents that inhibit monoamine oxidase (MAO) may increase the reinforcing value of low doses of nicotine. The aim of the present experiments was to further characterize the impact of MAO inhibition on the primary reinforcing and reinforcement enhancing effects of nicotine in rats. In a series of experiments, rats responded for intravenous nicotine infusions or a moderately-reinforcing visual stimulus in daily 1-h sessions. Rats received pre-session injections of known MAO inhibitors. The results show that (1) tranylcypromine (TCP), a known MAO inhibitor, increases sensitivity to the primary reinforcing effects of nicotine, shifting the dose-response curve for nicotine to the left, (2) inhibition of MAO-A, but not MAO-B, increases low-dose nicotine self-administration, (3) partial MAO-A inhibition, to the degree observed in chronic cigarette smokers, also increases low-dose nicotine self-administration, and (4) TCP decreases the threshold nicotine dose required for reinforcement enhancement. The results of the present experiments suggest cigarette smoke constituents that inhibit MAO-A, in the range seen in chronic smokers, are likely to increase the primary reinforcing and reinforcement enhancing effects of low doses of nicotine. If the FDA reduces the nicotine content of cigarettes, then variability in constituents that inhibit MAO-A could impact smoking.

The tendency to sign-track predicts cue-induced reinstatement during nicotine self-administration, and is enhanced by nicotine but not ethanol

RATIONALE

Some individuals are particularly responsive to reward-associated stimuli ("cues"), including the effects of these cues on craving and relapse to drug-seeking behavior. In the cases of nicotine and alcohol, cues may acquire these abilities via the incentive-enhancing properties of the drug.

OBJECTIVES

To determine the interaction between cue-responsivity and nicotine reinforcement, we studied the patterns of nicotine self-administration in rats categorized based on their tendency to approach a food-predictive cue ("sign-trackers") or a reward-delivery location ("goal-trackers"). In a second experiment, we determined whether nicotine and ethanol altered the incentive value of a food cue.

METHODS

Rats were classified as sign- or goal-trackers during a Pavlovian conditioned approach paradigm. Rats then self-administered intravenous nicotine (0.03 mg/kg infusions) followed by extinction and cue-induced reinstatement tests. We also tested the effects of nicotine (0.4 mg/kg base s.c.) or ethanol (0.7 g/kg i.p.) on the approach to, and reinforcing efficacy of, a food cue.

RESULTS

Sign-trackers showed greater reinstatement in response to a nicotine cue. Further, nicotine enhanced sign-tracking but not goal-tracking to a food cue and also enhanced responding for the food cue during the conditioned reinforcement test. Conversely, ethanol reduced sign-tracking and increased goal-tracking, but had no effect on conditioned reinforcement.

CONCLUSIONS

Our studies demonstrate that the tendency to attribute incentive value to a food cue predicts enhanced cue-induced reinstatement. Additionally, the incentive value of food cues is differentially modulated by nicotine and ethanol, which may be related to the reinforcing effects of these drugs.

Nicotine Enhances Footshock- and Lithium Chloride-Conditioned Place Avoidance in Male Rats

INTRODUCTION

Numerous studies have shown that nicotine (NIC) can enhance the reinforcing effects of non-NIC stimuli through a nonassociative mechanism. To date, it is unclear whether NIC reinforcement enhancement serves to increase behaviors motivated by rewarding stimuli only, or whether NIC potentiates behavior motivated by all stimuli, regardless of valence.

METHODS

The current study used a place conditioning procedure to examine whether acute NIC injection modulates avoidance of an environment previously associated with an aversive stimulus. Separate groups of rats underwent place conditioning using either lithium chloride (125mg/kg/ml, i.p.) or footshock (0.75 mA) as the aversive stimulus. Other rats served as nonconditioned controls. The magnitude of place avoidance was assessed after acute NIC (0.1 or 0.4mg/kg/ml, s.c.) or saline.

RESULTS

Rats avoided chambers previously paired with either lithium chloride or footshock, and conditioned place avoidance was significantly enhanced by NIC pre-treatment.

CONCLUSIONS

These results demonstrate that the ability of NIC to enhance motivated behavior extends to behaviors elicited by aversive stimuli, evidence that NIC affects behavior motivated by a broader range of stimuli than previously appreciated.

IMPLICATIONS

The current study examined whether the reinforcement enhancement properties of NIC apply to aversive stimuli by testing NIC enhancement of conditioned place avoidance in rats. The results demonstrate that NIC enhances the motivational impact of these distinct aversive stimuli, providing novel evidence that NIC affects behavior motivated by a broader range of stimuli than has previously been demonstrated.

Socially acquired nicotine self-administration with an aversive flavor cue in adolescent female rats

RATIONALE

Establishing a behavioral model for the effect of social environment on nicotine intake in rodents facilitates the investigation of molecular mechanisms critical for the interaction between social environment and cigarette smoking.

OBJECTIVES

Our main objective was to test the hypothesis that nicotine is the primary reinforcer in the socially acquired nicotine intravenous self-administration (IVSA) model by using an aversive flavor cue.

METHODS

Adolescent female rats were placed in operant conditioning chambers equipped with two lickometers. Operant licking triggered concurrent deliveries of a flavor (i.e., taste and odor) cue containing either quinine or saccharin and an i.v. infusion (30 μg/kg nicotine or saline). An audiovisual cue was provided for some groups of rats. A second rat that did not receive nicotine was placed in the operant conditioning chambers to provide either a neutral or an inducing (i.e., by consuming the flavored solution) social environment. These two rats were separated by a divider that allowed orofacial interactions.

RESULTS

Rats acquired stable nicotine IVSA with either the aversive or the appetitive flavor cue in the inducing social environment, and obtained similar amounts of infusions. The neutral social environment did not support nicotine IVSA with either cue. The audiovisual cue per se did not support nicotine IVSA but enhanced nicotine intake. Nicotine increased the number of concurrent nose pokes by the two rats into the center divider, a measure of social interaction.

CONCLUSIONS

Despite its aversive effects, nicotine is the primary reinforcer for the operant responses in the socially acquired nicotine IVSA model.

Design and development of a modified runway model of mouse drug self-administration

The present study established a novel mouse model of a runway drug self-administration in our laboratory. The operant runway apparatus consisted of three long runways arranged in a zig-zag manner. The methodology consisted of six distinct phases: habituation, preconditioning, conditioning, post-conditioning, extinction and reinstatement. The effects of saline were compared with escalating doses of either ethanol (0.5–4.0 g/kg, i.p), heroin (5–40 mg/kg, i.p), or nicotine (0.1–0.5mg/kg, i.p) administered in the goal box during the conditioning phase (day 1 to day 5). A significant decrease in the time of trained (conditioned) mice to reach the goal box confirmed the subjects’ motivation to seek those drugs on day 6 (expression). The mice were then subjected to non-rewarded extinction trials for 5 days over which run times were significantly increased. After 5 days of abstinence, a priming dose of ethanol or heroin (1/5th of maximum dose used in conditioning) significantly reinstated the drug-seeking behavior. These results suggest that the modified runway model can serve as a powerful behavioral tool for the study of the behavioral and neurobiological bases of drug self-administration and, as such, is appropriate simple but powerful tool for investigating the drug-seeking behavior of laboratory mice.

Adolescent Rats Self-Administer Less Nicotine Than Adults at Low Doses

INTRODUCTION

Although nearly 90% of current smokers initiated tobacco use during adolescence, little is known about reinforcement by nicotine in adolescents. Researchers are currently investigating whether a potential public health policy setting a tobacco product standard with very low nicotine levels would improve public health, and it is essential to understand whether data generated in adults translates to adolescents, particularly as it relates to the threshold dose of nicotine required to support smoking. The present study compared self-administration of low doses of nicotine between adolescent and adult rats.

METHODS

Adolescent (postnatal day [P] 30) and adult (P90) male and female rats were allowed to nosepoke to receive intravenous infusions of nicotine (3-100 μg/kg/infusion) during 16 daily 1-hour sessions.

RESULTS

At 10 μg/kg/infusion nicotine, adolescent rats earned significantly fewer infusions than adults. When responding for 30 μg/kg/infusion nicotine, rats of both ages earned a similar number of infusions; however, there were subtle differences in the distribution of infusions across the 1-hour session. No sex differences were apparent in either age group at any dose.

CONCLUSIONS

These results demonstrate that adolescent rats are less sensitive than adults to the primary reinforcing effects of nicotine. However, at nicotine doses that support self-administration in both age groups, adolescent and adult rats do not differ in acquisition or number of infusions earned. These results suggest that reducing nicotine levels in cigarettes to a level that does not support smoking in adults may be sufficient to reduce the acquisition of smoking in adolescents.

IMPLICATIONS

The results of the present studies demonstrate that adolescent rats are less sensitive than adults to the primary reinforcing effects of nicotine. These results suggest that reducing nicotine levels in cigarettes to a level that does not support smoking in adults will be sufficient to reduce the acquisition of smoking in adolescents.

Pharmacological and non-pharmacological factors that regulate the acquisition of ketamine self-administration in rats

BACKGROUND

Recreational ketamine use may be modulated by factors such as ketamine infusion patterns, associated conditioned stimuli and spatial-temporal contexts. Our aim was to study the pharmacological and non-pharmacological factors that regulate the acquisition of ketamine use.

METHODS

In experiment 1, four groups of male rats were trained to self-administer ketamine during nine 1-h daily sessions, under four reinforcement schedules: i) pre-session ketamine priming (Priming-[KET]), ii) conditioned stimulus (CS) paired to the ketamine infusions ([KET + CS]), iii) neither priming nor CS ([KET]), iv) combination of both (Priming-[KET + CS]). In experiment 2, two groups of male rats were trained to self-administer ketamine during nine 1-h daily or weekly sessions, under the Priming-[KET + CS] schedule. Lever pressing was then extinguished by saline substitution for ketamine infusion. Afterwards, ketamine was made available again upon responding under the same schedule.

RESULTS

The Priming-[KET + CS] schedule of reinforcement showed a significant increase in the number of ketamine reinforcements and a significant discrimination between active vs. inactive levers. The same schedule allowed the establishment of ketamine self-administration on a weekly basis. During the extinction phase, rate of responding significantly dropped in both weekly and daily groups although it was twofold longer in the former, which showed a lack of reacquisition.

CONCLUSIONS

Both pre-session ketamine priming and a conditioned stimulus paired to the ketamine infusions are required for the acquisition of ketamine self-administration. The longer extinction and the lack of reacquisition in the weekly group could be due to changes in temporal context that might affect the conditioning process.

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 MAO inhibition and a combination of minor alkaloids, β-carbolines, and acetaldehyde on nicotine self-administration in adult male rats

INTRODUCTION

Although nicotine is the primary reinforcing constituent in cigarettes, there is evidence that other constituents in cigarette smoke may interact with nicotine to reinforce smoking behavior.

METHODS

The present experiments investigated whether a novel combination of these cigarette smoke constituents would increase nicotine self-administration in adult male rats. The constituents included five minor alkaloids (anabasine, nornicotine, cotinine, myosmine, and anatabine), two β-carbolines (harman and norharman), and acetaldehyde. All doses were indexed to be proportional to concentrations in cigarette smoke given a standard dose of nicotine used in rodent self-administration, or ten times higher than this standard. To model MAO inhibition seen in chronic smokers, some groups received separate injections of tranylcypromine prior to each self-administration session.

RESULTS

Tranylcypromine increased low-dose nicotine self-administration independent of other smoke constituents, which had no effect on self-administration behavior. The effect of tranylcypromine was confirmed across a large range of reinforcement schedules. The effect of tranylcypromine on low-dose nicotine self-administration was observed regardless of whether the injection was delivered 1-h or 23-h prior to the self-administration session, consistent with the interpretation that MAO inhibition was responsible for the increase in self-administration, instead of acute off-target effects.

CONCLUSIONS

These data suggest that this cocktail of constituents does not significantly alter the primary reinforcing effects of nicotine, but constituents that inhibit MAO may increase the primary reinforcing effects of nicotine, especially at low doses.

Behavioral effects of phencyclidine on nicotine self-administration and reinstatement in the presence or absence of a visual stimulus in rats

RATIONALE

Tobacco use is a serious health problem in the USA, and this problem is potentiated in patients with schizophrenia. The reward system is implicated in schizophrenia and may contribute to the high comorbidity between nicotine use and schizophrenia, but very little research has been done on the topic. The reward-enhancement effect of nicotine has been shown to be important in nicotine use, but there have been no studies on this effect in animal models of schizophrenia.

OBJECTIVES

This study was designed to determine the effects of phencyclidine, used to model negative symptoms of schizophrenia, on self-administration of nicotine with or without a co-occurring sensory reinforcer [i.e., visual stimulus (VS)] in rats.

METHODS

Phencyclidine (2.0 mg/kg) was administered before each of seven nicotine self-administration sessions (0.01 mg/kg/inf) after which rats (n = 8-9 per group) were given 7 days of extinction without phencyclidine pretreatment. Reinstatement using phencyclidine (2.0 mg/kg), nicotine (0.2 mg/kg), and yohimbine (1.25 mg/kg, a pharmacological stressor) was tested after extinction to determine if previous exposure to phencyclidine would alter reinstatement of active lever pressing.

RESULTS

Phencyclidine initially decreased nicotine self-administration but only in the groups with a concurrent VS. This decrease in self-administration dissipated after 5 days. During reinstatement, rats that had previously received phencyclidine during self-administration with a VS were more sensitive to stress-induced reinstatement than any other group.

CONCLUSIONS

These results show a transitory effect of phencyclidine on nicotine self-administration. Phencyclidine may induce a potential sensitivity to pharmacological stressors contributing to reinstatement of nicotine.