Nicotine physical dependence and tolerance in the mouse following chronic oral administration

Translational research in nicotine dependence

Nicotine addiction accounts for 4.9 million deaths each year. Furthermore, although smoking represents a significant health burden in the United States, at present there are only three FDA-approved pharmacotherapies currently on the market: (1) nicotine replacement therapy, (2) bupropion, and (3) varenicline. Despite this obvious gap in the market, the complexity of nicotine addiction in addition to the increasing cost of drug development makes targeted drug development prohibitive. Furthermore, using combinations of mouse and human studies, additional treatments could be developed from off-the-shelf, currently approved medication lists. This article reviews translational studies targeting manipulations of the cholinergic system as a viable therapeutic target for nicotine addiction.

Glutamatergic and GABAergic metabolism in mouse brain under chronic nicotine exposure: implications for addiction

Background and Purpose

The effects of nicotine on cerebral metabolism and its influence on smoking behavior is poorly understood. An understanding of the chronic effects of nicotine on excitatory and inhibitory metabolic demand, and corresponding neurotransmission may provide clues for designing strategies for the optimal smoking cessation intervention. The objective of the current study was to investigate neuronal and astroglial metabolism in mice exposed to nicotine (0.5 and 2.0 mg/kg, sc) three times in a day for 4 weeks.

Experimental Approach/Principal Findings

Metabolic measurements were carried out by co-infusing [U-¹³C₆]glucose and [2-¹³C]acetate, and monitoring ¹³C labeling of amino acids in brain tissue extract using ¹H-[¹³C] and ¹³C-[¹H]-NMR spectroscopy. Concentration of ¹³C-labeled glutamate-C4 was increased significantly from glucose and acetate with chronic nicotine treatment indicating an increase in glucose oxidation by glutamatergic neurons in all brain regions and glutamate-glutamine neurotransmitter cycle in cortical and subcortical regions. However, chronic nicotine treatment led to increased labeling of GABA-C2 from glucose only in the cortical region. Further, increased labeling of glutamine-C4 from [2-¹³C]acetate is suggestive of increased astroglial activity in subcortical and cerebellum regions of brain with chronic nicotine treatment.

Conclusions and Significance

Chronic nicotine exposure enhanced excitatory activity in the majority of brain regions while inhibitory and astroglial functions were enhanced only in selected brain regions.

Bupropion and its main metabolite reverse nicotine chronic tolerance in the mouse

INTRODUCTION

Although the antidepressant bupropion is prescribed to aid in smoking cessation, little is known concerning its mechanisms of action in this regard. One factor that might influence quit success is nicotine tolerance, which could promote high levels of nicotine intake in order to maintain nicotine's subjective effects (thereby making attempts to reduce smoking more difficult).

METHODS

To explore whether bupropion and its active hydroxymetabolite modulate nicotine tolerance, mice were injected for 14 days with saline or nicotine. On Day 14, animals received saline, (2S,3S)-hydroxybupropion, or bupropion at different doses. On Day 15, mice were assayed on test day for nicotine-induced analgesia and hypothermia.

RESULTS

Animals chronically injected with saline + nicotine developed tolerance to nicotine's effects in both assays. Administration of bupropion and (2S,3S)-hydroxybupropion dose-dependently reversed chronic nicotine tolerance.

CONCLUSIONS

These results indicate that bupropion's ability to promote smoking cessation may be partly due to its attenuation of nicotine tolerance since both measured responses of nicotine (antinociception and hypothermia) are mediated to a large extent by neuronal α4β2* nicotine receptors.

Adult mice voluntarily progress to nicotine dependence in an oral self-selection assay

Nicotine has both rewarding and aversive properties in rodents, as shown by intravenous self-administration, intracranial self-stimulation, and conditioned place preference experiments. However, high throughput models of nicotine reward have not been developed in mice. In previous two-bottle studies, mice often chose to drink less from the nicotine bottle than from the water bottle, which raises the question whether these paradigms provide a model of the reinforcing properties of oral nicotine. We hypothesized that previous two-bottle choice paradigms included factors (such as the brief duration of trials, the addition of flavorings to both bottles, water bottles located relatively close to each other, etc.) that may have obstructed the formation of a learned association between the taste of nicotine and its delayed pharmacological effects. Here we show that a paradigm designed to simplify the acquisition of a learned association resulted in nicotine consumption by various strains and sexes that diverged progressively over a period of seven weeks. The strain and sex with the highest nicotine consumption (C57BL/6J females) showed steady and statistically significant increases in nicotine consumption throughout this period. C57BL/6J females were clearly responding to the reinforcing properties of nicotine because they chose to drink over 70% of their fluids from the nicotine bottle. Moreover, they became nicotine dependent, as shown by highly significant nicotine withdrawal symptoms after the nicotine bottle was removed. The strain and sex with the lowest consumption (A/J males) showed a significant decrease in nicotine consumption, and by the end of the experiment were drinking only 24% of their fluids from the nicotine bottle.

Developmental effects of acute, chronic, and withdrawal from chronic nicotine on fear conditioning

Pre-adolescence and adolescence are developmental periods associated with increased vulnerability for tobacco addiction, and exposure to tobacco during these periods may lead to long-lasting changes in behavioral and neuronal plasticity. The present study examined the short- and long-term effects of nicotine and nicotine withdrawal on fear conditioning in pre-adolescent, adolescent, and adult mice, and potential underlying substrates that may mediate the developmental effects of nicotine, such as changes in nicotinic acetylcholine receptor (nAChR) binding, CREB expression, and nicotine metabolism. Age-related differences existed in sensitivity to the effects of acute nicotine, chronic nicotine and nicotine withdrawal on contextual fear conditioning (no changes in cued fear conditioning were seen); younger mice were more sensitive to the acute effects and less sensitive to the effects of nicotine withdrawal 24 h post treatment cessation. Developmental differences in nAChR binding were associated with the effects of nicotine withdrawal on contextual learning. Developmental differences in nicotine metabolism and CREB expression were also observed, but were not related to the effects of nicotine withdrawal on contextual learning 24 h post treatment. Chronic nicotine exposure during pre-adolescence or adolescence, however, produced long-lasting impairments in contextual learning that were observed during adulthood, whereas adult chronic nicotine exposure did not. These developmental effects could be related to changes in CREB. Overall, there is a developmental shift in the effects of nicotine on hippocampus-dependent learning and developmental exposure to nicotine results in adult cognitive deficits; these changes in cognition may play an important role in the development and maintenance of nicotine addiction.

Withdrawal from chronic nicotine exposure alters dopamine signaling dynamics in the nucleus accumbens

BACKGROUND

Unaided attempts to quit smoking commonly fail during the first 2 weeks of the nicotine withdrawal syndrome. Alterations in dopamine (DA) signaling correlate with withdrawal from chronic nicotine exposure, but those changes have not been well-characterized.

METHODS

Mice were administered nicotine in their drinking water for 4 or 12 weeks. Then nicotine was withheld for 1 to 10 days while DA signaling was characterized with in vivo microdialysis or fast-scan cyclic voltammetry.

RESULTS

Upon withdrawal of nicotine, the basal DA concentration in the nucleus accumbens decreased as measured by microdialysis. The length of time that the low basal DA state lasted depended on the length of the chronic nicotine treatment. Microdialysis indicated that acute re-exposure to nicotine during withdrawal temporarily reversed this hypodopaminergic state. Voltammetry measurements supported the microdialysis results by showing that nicotine withdrawal decreased tonic and phasic DA release. The basal DA concentration and tonic DA signals, however, were disproportionately lower than the phasic DA signals. Therefore, the phasic/tonic DA signaling ratio was increased during the withdrawal period.

CONCLUSIONS

The relative increase in the sensitivity of DA release to phasic stimulation suggests an increase in the signal-to-noise relationship of DA signaling during the withdrawal period. Therefore, the DA signal produced by acute nicotine re-exposure produces a DA response that might reinforce relapse to drug use (i.e., smoking). Because the basal DA concentration is low during withdrawal, therapies aimed at elevating the background DA signal represent a reasonable treatment strategy for nicotine-dependent individuals attempting to quit.

Pharmacogenetics of smoking cessation: role of nicotine target and metabolism genes

Many smokers attempt to quit smoking but few are successful in the long term. The heritability of nicotine addiction and smoking relapse have been documented, and research is focused on identifying specific genetic influences on the ability to quit smoking and response to specific medications. Research in genetically modified cell lines and mice has identified nicotine acetylcholine receptor subtypes that mediate the pharmacological and behavioral effects of nicotine sensitivity and withdrawal. Human genetic association studies have identified single nucleotide polymorphisms (SNPs) in genes encoding nicotine acetylcholine receptor subunits and nicotine metabolizing enzymes that influence smoking cessation phenotypes. There is initial promising evidence for a role in smoking cessation for SNPs in the β2 and α5/α3/β4 nAChR subunit genes; however, effects are small and not consistently replicated. There are reproducible and clinically significant associations of genotypic and phenotypic measures of CYP2A6 enzyme activity and nicotine metabolic rate with smoking cessation as well as response to nicotine replacement therapies and bupropion. Prospective clinical trials to identify associations of genetic variants and gene-gene interactions on smoking cessation are needed to generate the evidence base for both medication development and targeted therapy approaches based on genotype.

Electrophysiological correlates of associative learning in smokers: a higher-order conditioning experiment

BACKGROUND

Classical conditioning has been suggested to play an important role in the development, maintenance, and relapse of tobacco smoking. Several studies have shown that initially neutral stimuli that are directly paired with smoking are able to elicit conditioned responses. However, there have been few human studies that demonstrate the contribution of higher-order conditioning to smoking addiction, although it is assumed that higher-order conditioning predominates learning in the outside world. In the present study a higher-order conditioning task was designed in which brain responses of smokers and non-smokers were conditioned by pairing smoking-related and neutral stimuli (CS1smoke and CS1neutral) with two geometrical figures (CS2smoke and CS2neutral). ERPs were recorded to all CSs.

RESULTS

Data showed that the geometrical figure that was paired with smoking stimuli elicited significantly larger P2 and P3 waves than the geometrical figure that was paired with neutral stimuli. During the first half of the experiment this effect was only present in smokers whereas non-smokers displayed no significant differences between both stimuli, indicating that neutral cues paired with motivationally relevant smoking-related stimuli gain more motivational significance even though they were never paired directly with smoking. These conclusions are underscored by self-reported evidence of enhanced second-order conditioning in smokers.

CONCLUSIONS

It can be concluded that smokers show associative learning for higher-order smoking-related stimuli. The present study directly shows the contribution of higher-order conditioning to smoking addiction and is the first to reveal its electrophysiological correlates. Although results are preliminary, they may help in understanding the etiology of smoking addiction and its persistence.

Chronic oral nicotine increases brain [3H]epibatidine binding and responsiveness to antidepressant drugs, but not nicotine, in the mouse forced swim test

INTRODUCTION

Smoking rates among depressed individuals is higher than among healthy subjects, and nicotine alleviates depressive symptoms. Nicotine increases serotonergic and noradrenergic neuronal activity and facilitates serotonin and noradrenaline release. In mice, acute nicotine administration enhances the activity of antidepressants in the mouse forced swim (mFST) and tail suspension tests. Here, we investigated if this action of nicotine is also reflected in a chronic treatment regimen.

MATERIALS AND METHODS

After chronic treatment with nicotine in the drinking water, mice were challenged with nicotine, duloxetine, citalopram, and reboxetine in the mFST. Additionally, 8-OH-DPAT- and clonidine-induced hypothermia was tested in vehicle- and nicotine-pretreated mice, as a measure of 5-HT(1A) and alpha(2)-adrenoceptor function, respectively. Finally, the effects on the brain expression levels of high- and low-affinity nicotinic acetylcholine receptors (nAChRs) and the transporters for serotonin (SERT) and noradrenaline (NET) were assessed using [(3)H]epibatidine, [(3)H]alpha-bungarotoxin, [(3)H]citalopram, and [(3)H]nisoxetine binding, respectively.

RESULTS

In the mFST, nicotine-pretreated mice did not show altered response to the nicotine challenge, but increased responses to all three antidepressants tested were observed when compared to mice that had been administered drinking water without nicotine. There was no change in hypothermic responses to 8-OH-DPAT or clonidine. [(3)H]epibatidine binding was significantly increased in all brain regions investigated; whereas, [(3)H]alpha-bungarotoxin, [(3)H]citalopram, and [(3)H]nisoxetine binding were not altered, indicating that chronic oral nicotine increases the expression and/or affinity of high-affinity nAChRs, but not low-affinity nAChRs, SERT, or NET.

DISCUSSION

It is suggested that the increased sensitivity to antidepressants after chronic nicotine exposure involves increased high-affinity nAChR-mediated neurotransmission.

Mechanism-based medication development for the treatment of nicotine dependence

Tobacco use is a global problem with serious health consequences. Though some treatment options exist, there remains a great need for new effective pharmacotherapies to aid smokers in maintaining long-term abstinence. In the present article, we first discuss the neural mechanisms underlying nicotine reward, and then review various mechanism-based pharmacological agents for the treatment of nicotine dependence. An oversimplified hypothesis of addiction to tobacco is that nicotine is the major addictive component of tobacco. Nicotine binds to alpha4beta2 and alpha7 nicotinic acetylcholine receptors (nAChRs) located on dopaminergic, glutamatergic and GABAergic neurons in the mesolimbic dopamine (DA) system, which causes an increase in extracellular DA in the nucleus accumbens (NAc). That increase in DA reinforces tobacco use, particularly during the acquisition phase. Enhanced glutamate transmission to DA neurons in the ventral tegmental area appears to play an important role in this process. In addition, chronic nicotine treatment increases endocannabinoid levels in the mesolimbic DA system, which indirectly modulates NAc DA release and nicotine reward. Accordingly, pharmacological agents that target brain acetylcholine, DA, glutamate, GABA, or endocannabonoid signaling systems have been proposed to interrupt nicotine action. Furthermore, pharmacokinetic strategies that alter plasma nicotine availability, metabolism and clearance also significantly alter nicotine's action in the brain. Progress using these pharmacodynamic and pharmacokinetic agents is reviewed. For drugs in each category, we discuss the mechanistic rationale for their potential anti-nicotine efficacy, major findings in preclinical and clinical studies, and future research directions.

Rodent models of nicotine withdrawal syndrome

Simple, rapid and inexpensive rodent models of nicotine physical dependence and withdrawal syndrome have proved useful for preliminary screening of smoking cessation treatments. They have led to an exponential increase of knowledge regarding the underlying neurobiological mechanisms of dependence and withdrawal syndrome. The human nicotine withdrawal syndrome in smoking cessation is variable and multidimensional, involving irritability, anxiety, depression, cognitive and attentional impairments, weight gain, sleep disturbances, and craving for nicotine. Aside from sleep disturbances, analogous phenomena have been seen in rodent models using different measures of withdrawal intensity. It appears likely that different withdrawal phenomena may involve some partially divergent mechanisms. For example, depression-like phenomena may involve alterations in mechanisms such as the mesolimbic dopamine pathway from the ventral tegmental area to the nucleus accumbens. Irritability and anxiety may involve alterations in endogenous opioid systems and other regions, such as the amygdala. This chapter reviews many additional anatomical, neurochemical, and developmental elements that impact nicotine physical dependence.

Involvement of kappa/dynorphin system in the development of tolerance to nicotine-induced antinociception

The aim of the present study was to explore the possible role of kappa/dynorphin system in the development of tolerance to nicotine antinociception in mice. First, we observed that kappa-opioid receptor (KOP-r) participates in the acute spinal antinociception produced by nicotine (3 and 5 mg/kg, s.c.) since the pre-treatment with the selective kappa antagonist nor-binaltorphimine (3 mg/kg, i.p.) attenuated this response in the tail-immersion test but not in the hot-plate test nor in locomotor responses. Possible changes in the expression of KOP-r were investigated in tolerant mice to nicotine antinociception by using autoradiography of [3H]CI-977 binding. The density of KOP-r decreased in the spinal cord of tolerant mice. In addition, bi-directional cross-tolerance between nicotine (3 and 5 mg/kg, s.c.) and the selective kappa agonist U50,488H (10 mg/kg, s.c.) was found in the tail-immersion test. Recent evidences indicate that an up-regulation of dynorphin levels in the spinal cord and subsequent activation of NMDA receptors participate in the development of tolerance to opioid and cannabinoid antinociception. In this study, dynorphin content in the lumbar spinal cord was similar in control and nicotine tolerant mice. Furthermore, the administration of the NMDA antagonist MK-801 (0.03 and 0.01 mg/kg, i.p.) before each daily nicotine injection did not modify the development of nicotine tolerance. In summary, these data indicate that KOP-r is directly involved in the development of tolerance to nicotine antinociception by a mechanism independent from dynorphin and NMDA receptors.

Genetics of nicotinic acetylcholine receptors: Relevance to nicotine addiction

Human twin studies have suggested that there is a substantial genetic component underlying nicotine dependence, ongoing smoking and ability to quit. Similarly, animal studies have identified a number of genes and gene products that are critical for behaviors related to nicotine addiction. Classical genetic approaches, gene association studies and genetic engineering techniques have been used to identify the gene products involved in nicotine dependence. One class of genes involved in nicotine-related behavior is the family of nicotinic acetylcholine receptors (nAChRs). These receptors are the primary targets for nicotine in the brain. Genetic engineering studies in mice have identified a number of subunits that are critical for the ability of nicotine to activate the reward system in the brain, consisting of the dopaminergic cell bodies in the ventral tegmental area and their terminals in the nucleus accumbens and other portions of the mesolimbic system. In this review we will discuss the various lines of evidence suggesting that nAChRs may be involved in smoking behavior, and will review the human and animal studies that have been performed to date examining the genetic basis for nicotine dependence and smoking.

Long-term effects of chronic nicotine exposure on brain nicotinic receptors

Chronic nicotine exposure results in long-term homeostatic regulation of nicotinic acetylcholine receptors (nAChRs) that play a key role in the adaptative cellular processes leading to addiction. However, the relative contribution of the different nAChR subunits in this process is unclear. Using genetically modified mice and pharmacological manipulations, we provide behavioral, electrophysiological, and pharmacological evidence for a long-term mechanism by which chronic nicotine triggers opposing processes differentially mediated by beta2*- vs. alpha7*nAChRs. These data offer previously undescribed insights into the understanding of nicotine addiction and the treatment of several human pathologies by nicotine-like agents chronically acting on beta2*- or alpha7*nAChRs.

Guidelines on nicotine dose selection for in vivo research

RATIONALE

This review provides insight for the judicious selection of nicotine dose ranges and routes of administration for in vivo studies. The literature is replete with reports in which a dosaging regimen chosen for a specific nicotine-mediated response was suboptimal for the species used. In many cases, such discrepancies could be attributed to the complex variables comprising species-specific in vivo responses to acute or chronic nicotine exposure.

OBJECTIVES

This review capitalizes on the authors' collective decades of in vivo nicotine experimentation to clarify the issues and to identify the variables to be considered in choosing a dosaging regimen. Nicotine dose ranges tolerated by humans and their animal models provide guidelines for experiments intended to extrapolate to human tobacco exposure through cigarette smoking or nicotine replacement therapies. Just as important are the nicotine dosaging regimens used to provide a mechanistic framework for acquisition of drug-taking behavior, dependence, tolerance, or withdrawal in animal models.

RESULTS

Seven species are addressed: humans, nonhuman primates, rats, mice, Drosophila, Caenorhabditis elegans, and zebrafish. After an overview on nicotine metabolism, each section focuses on an individual species, addressing issues related to genetic background, age, acute vs chronic exposure, route of administration, and behavioral responses.

CONCLUSIONS

The selected examples of successful dosaging ranges are provided, while emphasizing the necessity of empirically determined dose-response relationships based on the precise parameters and conditions inherent to a specific hypothesis. This review provides a new, experimentally based compilation of species-specific dose selection for studies on the in vivo effects of nicotine.

Attenuation of nicotine-induced rewarding effects in A2A knockout mice

The non-selective A2A antagonist caffeine has been reported to modify nicotine-induced locomotor and reinforcing effects. In the present study, we have investigated the specific role of A2A adenosine receptors in the behavioural responses induced by nicotine by using genetically modified mice lacking A2A adenosine receptors. Acute nicotine administration induced a similar decrease of locomotor activity in A2A knockout mice and wild-type littermates. Acute antinociceptive responses elicited by nicotine in the tail-immersion and hot-plate tests were unaffected in these mutant mice. The rewarding properties of nicotine were then investigated using the place-conditioning paradigm. Nicotine-induced conditioned place preference was suppressed in A2A knockout mice. Accordingly, in vivo microdialysis studies revealed that the extracellular levels of dopamine in the nucleus accumbens were not increased after nicotine administration in mutant mice. Wild-type and A2A knockout mice were trained in conditioned taste aversion procedure in which drinking a saccharin or saline solution was paired with nicotine or saline injections. A similar reduction in the intake of nicotine-paired solution in this paradigm was obtained in both genotypes. Finally, the administration of the nicotinic antagonist mecamylamine in nicotine-dependent mice precipitated a similar withdrawal syndrome in both genotypes. Together, the present results identify A2A adenosine receptors as an important factor that contributes to the rewarding properties of nicotine.

Mu-opioid receptors are involved in the tolerance to nicotine antinociception

Several studies have shown the participation of the endogenous opioid system on the antinociceptive effects and addictive properties of nicotine. The aim of the present study was to explore the involvement of the mu-opioid receptors in the development of tolerance to nicotine antinociception. Chronic treatment of C57BL/6 mice with nicotine (5 mg/kg s.c., three times daily during 12 days) resulted in tolerance to its antinociceptive responses in the tail-immersion test. We investigated the possible existence of adaptive changes in the expression and/or functional activity of mu-opioid receptors in these tolerant mice by using autoradiography of [(3)H]D-Ala(2)-MePhe(4)-Gly-ol(5) enkephalin ([(3)H]DAMGO) binding and DAMGO-stimulated guanosine [(35)S]5'-(gamma-thio)-triphosphate ([(35)S]GTPgammaS) binding. The density of mu-opioid receptors in the spinal cord was not modified in nicotine-tolerant mice, whereas a decrease was found in the caudate-putamen, as well as in the core and the shell of the nucleus accumbens. However, the functional activity of these receptors was significantly increased in the spinal cord as a consequence of nicotine treatment. To further investigate the role of mu-opioid receptors in the tolerance to nicotine-induced antinociception, we evaluated this response in C57BL/6 mu-opioid receptor knockout mice. Chronic nicotine treatment produced tolerance in both wild-type and knockout animals, but tolerance developed faster in mice lacking mu-opioid receptors. These results indicate that mu-opioid receptors play an important role in the development of tolerance to nicotine antinociceptive effects.

Nicotine self-administration in mice is associated with rates of nicotine inactivation by CYP2A5

RATIONALE

Cyp2a5, the mouse homologue of human CYP2A6, encodes for the enzyme responsible for the primary metabolism of nicotine. Variation in human CYP2A6 activity can alter the amount smoked such as number of cigarettes smoked per day and smoking intensity. Different mouse strains self-administer different amounts of oral nicotine and quantitative trait loci analyses in mice suggested that Cyp2a5 may be involved in differential nicotine consumption behaviors.

OBJECTIVES

The goal of this study was to examine whether in vivo nicotine consumption levels were associated with CYP2A5 protein levels and in vitro nicotine metabolism in mice.

METHODS

F2 mice propagated from high (C57Bl/6) and low (St/bJ) nicotine consuming mice were analyzed for CYP2A5 hepatic protein levels and in vitro nicotine metabolizing activity.

RESULTS

We found that F2 male high-nicotine (n=8; 25.1+/-1.2 microg nicotine/day) consumers had more CYP2A5 protein, compared to low (n=11; 3.8+/-1.4 microg nicotine/day) consumers (10.2+/-1.0 vs 6.5+/-1.3 CYP2A5 units). High consumers also metabolized nicotine faster than the low consumers (6 microM: 0.18+/-0.04 vs 0.14+/-0.07; 30 microM: 0.36+/- 0.06 vs 0.26+/-0.13; 60 microM: 0.49+/-0.05 vs 0.32+/-0.17 nmol/min/mg). In contrast, female high- (25.1+/-2.1 microg nicotine/day) and low-nicotine (4.7+/-1.4 microg nicotine/day) consumers did not show pronounced differences in nicotine metabolism or CYP2A4/5 protein levels; this is consistent with other studies of sex differences in response to nicotine.

CONCLUSIONS

These data suggested that among male F2 mice, increased nicotine self-administration is associated with increased rates of nicotine metabolism, most likely, as a result of greater CYP2A5 protein levels.

beta2-Subunit-containing nicotinic acetylcholine receptors are involved in nicotine-induced increases in conditioned reinforcement but not progressive ratio responding for food in C57BL/6 mice

RATIONALE

Nicotine administration potentiates conditioned reinforcement in rats, an effect that persists for weeks after chronic exposure. Little is known regarding the nicotinic receptor subtypes that may mediate this effect.

OBJECTIVE

The purpose of this study was to determine whether beta2-subunit-containing nicotinic acetylcholine receptors (beta2*nAChRs) are necessary for lasting effects of nicotine on conditioned and primary reinforcement in mice.

METHODS

Beta2 knockout (beta2KO) and wild-type (WT) mice received 14 days of nicotine exposure (NIC, 200 microg/ml in 2% saccharin) or saccharin alone (SAC) in their drinking water. Five days later, mice received paired presentations of a conditioned stimulus (CS) with water unconditioned stimulus (US) or explicitly unpaired presentations of the CS and US during Pavlovian discriminative approach training. Training was followed by two conditioned reinforcement tests. Mice were subsequently tested for food-reinforced responding in the absence of explicit cues followed by a progressive ratio test.

RESULTS

During conditioned reinforcement testing, only mice in the paired condition showed increased responding in the CS-reinforced aperture over inactive apertures. WT-NIC mice showed enhanced conditioned reinforcement compared to WT-SAC animals. beta2KO-SAC mice showed elevated conditioned reinforcement compared to WT-SAC subjects, but beta2KO-NIC and beta2KO-SAC mice did not differ in responding with conditioned reinforcement. Prior nicotine exposure did not alter food-reinforced responding but resulted in elevated break points for food in both genotypes.

CONCLUSION

These data show that nicotine exposure enhances conditioned reinforcement in mice and indicate that beta2*nAChRs are necessary for nicotine-dependent enhancement of incentive aspects of motivation but not motivation for primary reinforcement measured by progressive ratio responding.

Nicotine physical dependence in the mouse: involvement of the alpha7 nicotinic receptor subtype

Although chronic nicotine produces dependence in mice, the role of specific nicotinic receptors has not been examined in knockout animals. The present study utilized alpha7 nicotinic receptor knockout mice to explore the role of this receptor subunit in nicotine dependence. Mice were chronically exposed to nicotine (0 or 200 microg/ml) in their drinking water and assayed for somatic withdrawal signs, hyperalgesia (tail-flick and hot-plate) and spontaneous activity following nicotine cessation. Nicotine withdrawal produced increased somatic signs in both strains and hyperalgesia in wild-type, but not in knockout animals. These results indicate that the alpha7 nicotinic receptor subunit may mediate some aspects of nicotine dependence.