Effects of nicotine withdrawal on central dopaminergic systems

Is impulsivity related to attentional bias in cigarette smokers? An exploration across levels of nicotine dependency and deprivation

Research has largely focused on how attentional bias to smoking-related cues and impulsivity independently influence the development and maintenance of cigarette smoking, with limited exploration of the relationship between these mechanisms. The current experiments systematically assessed relationships between multiple dimensions of impulsivity and attentional bias, at different stages of attention, in smokers varying in nicotine dependency and deprivation. Nonsmokers (NS; n  = 26), light-satiated smokers (LS; n  = 25), heavy-satiated smokers (HS; n  = 23) and heavy 12-hour nicotine-deprived smokers (HD; n  = 30) completed the Barratt Impulsivity Scale, delayed discounting task, stop-signal task, information sampling task and a visual dot-probe assessing initial orientation (200 ms) and sustained attention (2000 ms) toward smoking-related cues. Sustained attention to smoking-related cues was present in both HS and LS, while initial orientation bias was only evident in HS. HS and LS also had greater levels of trait motor and nonplanning impulsivity and heightened impulsive choice on the delay discounting task compared with NS, while heightened trait attentional impulsivity was only found in HS. In contrast, in HD, nicotine withdrawal was associated with no attentional bias but heightened reflection impulsivity, poorer inhibitory control and significantly lower levels of impulsive choice relative to satiated smokers. Trait and behavioral impulsivity were not related to the extent of attentional bias to smoking-related cues at any stage of attention, level of nicotine dependency or state of deprivation. Findings have both clinical and theoretical implications, highlighting the unique and independent roles impulsivity and attentional bias may play at different stages of the nicotine addiction cycle.

The involvement of dopamine and D2 receptor-mediated transmission in effects of cotinine in male rats

Previous studies indicated that cotinine, the major metabolite of nicotine, supported intravenous self-administration and exhibited relapse-like drug-seeking behaviors in rats. Subsequent studies started to reveal an important role of the mesolimbic dopamine system in cotinine's effects. Passive administration of cotinine elevated extracellular dopamine levels in the nucleus accumbens (NAC) and the D1 receptor antagonist SCH23390 attenuated cotinine self-administration. The objective of the current study was to further investigate the role of mesolimbic dopamine system in mediating cotinine's effects in male rats. Conventional microdialysis was conducted to examine NAC dopamine changes during active self-administration. Quantitative microdialysis and Western blot were used to determine cotinine-induced neuroadaptations within the NAC. Behavioral pharmacology was performed to investigate potential involvement of D2-like receptors in cotinine self-administration and relapse-like behaviors. NAC extracellular dopamine levels increased during active self-administration of cotinine and nicotine with less robust increase during cotinine self-administration. Repeated subcutaneous injections of cotinine reduced basal extracellular dopamine concentrations without altering dopamine reuptake in the NAC. Chronic self-administration of cotinine led to reduced protein expression of D2 receptors within the core but not shell subregion of the NAC, but did not change either D1 receptors or tyrosine hydroxylase in either subregion. On the other hand, chronic nicotine self-administration had no significant effect on any of these proteins. Systemic administration of eticlopride, a D2-like receptor antagonist attenuated both cotinine self-administration and cue-induced reinstatement of cotinine seeking. These results further support the hypothesis that the mesolimbic dopamine transmission plays a critical role in mediating reinforcing effects of cotinine.

GABAB receptors blockage modulates somatic and aversive manifestations induced by nicotine withdrawal

There is substantial evidence that GABA_B agonist, baclofen, prevents somatic and motivational responses induced by nicotine withdrawal and may target drug cue vulnerabilities in humans. In this context, we explored different aspects associated with the possible mechanisms whereby the GABA_B receptors might influence nicotine withdrawal. Male mice received nicotine (2.5 mg/kg, s.c.) 4 times daily, for 7 consecutive days. Nicotine-treated mice received the nicotinic acetylcholine receptor antagonist, mecamylamine (MEC, 2 or 3.5 mg/kg, s.c.), to precipitate the withdrawal state. A second group of dependent mice received 2-hydroxysaclofen (GABA_B receptor antagonist, 1 mg/kg, s.c.) before MEC-precipitated abstinence. Somatic signs of nicotine withdrawal were measured for 30 min. Anxiogenic-like response associated to nicotine withdrawal was assessed by the elevated plus maze test. The dysphoric/aversive effect induced by nicotine withdrawal was evaluated using conditioned place aversion paradigm. Dopamine, serotonin and its metabolites concentrations were determined by HPLC in the striatum, cortex and hippocampus. Finally, α4β2 nicotinic acetylcholine receptor density was determined in several brain regions using autoradiography assays. The results showed that MEC-precipitated nicotine withdrawal induced somatic manifestations, anxiogenic-like response and dysphoric/aversive effect, and 2-hydroxysaclofen potentiated these behavioral responses. Additionally, 2-hydroxysaclofen was able to change striatal dopamine levels and α4β2 nicotinic acetylcholine receptor density, both altered by MEC-precipitated nicotine withdrawal. These findings provide important contributions to elucidate neurobiological mechanisms implicated in nicotine withdrawal. We suggest that GABA_B receptor activity is necessary to control alterations induced by nicotine withdrawal, which supports the idea of targeting GABA_B receptors to treat tobacco addiction in humans.

Chronic Lorcaserin Treatment Reverses the Nicotine Withdrawal-Induced Disruptions to Behavior and Maturation in Developing Neurons in the Hippocampus of Rats

Preclinical data have shown that treatment with serotonin (5-HT)_2C receptor agonists inhibits the behavioral effects of nicotine, including self-administration, reinstatement, and locomotor responses to nicotine. Since the data on the effects of 5-HT_2C receptor agonism on nicotine withdrawal signs are limited, we aimed to investigate whether 5-HT_2C receptor agonism alleviated the behavioral and neurobiochemical (hippocampal neurogenesis) consequences of nicotine withdrawal in Sprague-Dawley rats. Our data indicate that withdrawal from nicotine self-administration induced locomotor hyperactivity, lengthened immobility time (the forced swim test), induced ‘drug-seeking’ behavior and deficits in cognition-like behavior (the novel object recognition task). A two-week exposure to the 5-HT_2C receptor agonist lorcaserin attenuated locomotor hyperactivity and induced recovery from depression-like behavior. Analyses of brain slices from nicotine-withdrawn animals revealed that lorcaserin treatment recovered the reduced number of doublecortin (DCX)-positive cells, but it did not affect the number of K_i-67- or 5-bromo-2’-deoxyuridine (BrdU)-positive cells or the maturation of proliferating neurons in drug-weaned rats. To summarize, we show that lorcaserin alleviated locomotor responses and depression-like state during nicotine withdrawal. We propose that the modulatory effect of lorcaserin on the ‘affective’ aspects of nicotine cessation may be linked to the positive changes caused by the compound in hippocampal neurogenesis during nicotine withdrawal.

Changes in striatal dopamine release and locomotor activity following acute withdrawal from chronic nicotine are mediated by CRF1, but not CRF2, receptors

The aim of the present study was to investigate the participation of corticotropin-releasing factor (CRF) receptors (CRF1 and CRF2) in the alterations of the dorsal and ventral striatal dopamine release and the vertical and horizontal locomotor activity observed in rats following chronic nicotine treatment and consequent acute withdrawal. In this purpose, male Wistar rats were exposed to repeated intraperitoneal (ip) injection with nicotine or saline solution for 7 days. On the 8th day or the 9th day the rats were injected intracerebroventricularly (icv) with selective CRF1 antagonist antalarmin or selective CRF2 antagonist astressin_2B or saline solution. Thirty minutes after the icv injection the changes of the horizontal and vertical locomotor activity were recorded in an in vivo conducta system. Immediately after the behavioral recordings the changes of the dorsal and ventral striatal dopamine release were determined in an in vitro superfusion system. On the 8th day, the horizontal and vertical locomotor activities and the dorsal and ventral striatal dopamine releases increased significantly in nicotine-treated rats, compared to the saline-treated ones. On the 9th day, the horizontal locomotor activity and the dorsal striatal dopamine release increased significantly, whereas the vertical locomotor activity and the ventral striatal dopamine release decreased significantly in nicotine-treated rats, compared to the saline-treated ones. All the changes observed were attenuated significantly by antalarmin, but not astressin_2B. The present study demonstrates that the changes of striatal dopamine release and locomotor activity observed following chronic nicotine treatment and consequent acute withdrawal are mediated by CRF1, but not CRF2, receptor.

A protocadherin gene cluster regulatory variant is associated with nicotine withdrawal and the urge to smoke

Nicotine withdrawal symptoms contribute to relapse in smokers, thereby prolonging the harm caused by smoking. To investigate the molecular basis for this phenomenon, we conducted a genome-wide association study of DSM-IV nicotine withdrawal in a sample of African American (AA) and European American (EA) smokers. A combined AA and EA meta-analysis (n=8021) identified three highly correlated single nucleotide polymorphisms (SNPs) in the protocadherin (PCDH)-α, -β and -γ gene cluster on chromosome 5 that were associated with nicotine withdrawal (P<5 × 10-8). We then studied one of the SNPs, rs31746, in an independent sample of smokers who participated in an intravenous nicotine infusion study that followed overnight smoking abstinence. After nicotine infusion, abstinent smokers with the withdrawal risk allele experienced greater alleviation of their urges to smoke, as assessed by the Brief Questionnaire on Smoking Urges (BQSU). Prior work has shown that the PCDH-α, -β and -γ genes are expressed in neurons in a highly organized manner. We found that rs31746 mapped to a long-range neuron-specific enhancer element shown previously to regulate PCDH-α, -β and -γ gene expression. Using Braincloud mRNA expression data, we identified a robust and specific association between rs31746 and PCDH-β8 mRNA expression in frontal cortex tissue (P<1 × 10-5). We conclude that PCDH-α, -β and -γ gene cluster regulatory variation influences the severity of nicotine withdrawal. Further studies on the PCDH-α, -β and -γ genes and their role in nicotine withdrawal may inform the development of novel smoking cessation treatments and reduce the harm caused by tobacco smoking.

Nicotinic receptor blockade decreases fos immunoreactivity within orexin/hypocretin-expressing neurons of nicotine-exposed rats

Tobacco smoking is the leading cause of preventable death in the United States. Nicotine is the principal psychoactive ingredient in tobacco that causes addiction. The structures governing nicotine addiction, including those underlying withdrawal, are still being explored. Nicotine withdrawal is characterized by negative affective and cognitive symptoms that enhance relapse susceptibility, and suppressed dopaminergic transmission from ventral tegmental area (VTA) to target structures underlies behavioral symptoms of nicotine withdrawal. Agonist and partial agonist therapies help 1 in 4 treatment-seeking smokers at one-year post-cessation, and new targets are needed to more effectively aid smokers attempting to quit. Hypothalamic orexin/hypocretin neurons send excitatory projections to dopamine (DA)-producing neurons of VTA and modulate mesoaccumbal DA release. The effects of nicotinic receptor blockade, which is commonly used to precipitate withdrawal, on orexin neurons remain poorly investigated and present an attractive target for intervention. The present study sought to investigate the effects of nicotinic receptor blockade on hypothalamic orexin neurons using mecamylamine to precipitate withdrawal in rats. Separate groups of rats were treated with either chronic nicotine or saline for 7-days at which point effects of mecamylamine or saline on somatic signs and anxiety-like behavior were assessed. Finally, tissue from rats was harvested for immunofluorescent analysis of Fos within orexin neurons. Results demonstrate that nicotinic receptor blockade leads to reduced orexin cell activity, as indicated by lowered Fos-immunoreactivity, and suggest that this underlying cellular activity may be associated with symptoms of nicotine withdrawal as effects were most prominently observed in rats given chronic nicotine. We conclude from this study that orexin transmission becomes suppressed in rats upon nicotinic receptor blockade, and that behavioral symptoms associated with nicotine withdrawal may be aided by intervention upon orexinergic transmission.

The effect of nicotine induced behavioral sensitization on dopamine D1 receptor pharmacology: An in vivo and ex vivo study in the rat

Behavioral sensitization is a phenomenon which can develop following repeated intermittent administration of a range of psychostimulants, and other compounds, and may model neuroplastic changes seen in addictive processes and neuropsychiatric disease. The aim of the present study was to investigate the effect of dopamine D1 receptor (D1R) ligands on nicotine-induced behavioral sensitization and their molecular consequences in the striatum. Wistar rats were chronically treated (5 days) with vehicle or nicotine (0.4 mg/kg; s.c.) and locomotor activity was measured. Following a 5 day withdrawal period, rats were pretreated with vehicle or the D1R antagonist SCH-23390 (0.03 mg/kg; i.p.) and challenged with nicotine. Either 45 min or 24h post-challenge, the striatum was isolated and ex vivo receptor binding and cAMP accumulation (using LC-MS/MS) were assessed. It was shown that chronic nicotine administration induced the development and expression of locomotor sensitization, of which the latter was blocked by SCH-23390. Nicotine-induced sensitization had no effect on forskolin stimulated cAMP accumulation but increased the efficacy of dopamine for the D1R and decreased the potency of D1R agonists. These effects were antagonized by in vivo pre-challenge with SCH-23390. No effect on D1 receptor binding was observed. Moreover, time dependent effects were observed between tissue taken 45 min and 24h post-challenge. The present findings provide a connection between behavioral sensitization and intracellular cAMP accumulation through the D1R. Together these data suggest that changes in D1R signaling in the dorsal striatum may play an important role in the underlying mechanisms of nicotine-induced behavioral sensitization.

Effects of acute and repeated nicotine administration on delay discounting in Lewis and Fischer 344 rats

Biological differences may underlie individual differences in impulsive behavior, such as choice for a smaller, more immediate reinforcer over a larger, more delayed reinforcer. Repeated exposure to drugs of abuse may have different effects on such behavior. To evaluate the acute and repeated effects of nicotine on impulsive choice, two strains of rats that have been shown to differ in impulsive choice were tested in a delay-discounting paradigm. Eight Lewis and eight Fischer 344 rats were allowed to choose between one food pellet delivered immediately and three food pellets delivered after a delay. The delay systematically increased in blocks of trials within each session, and the delay value at which the choice for the two alternatives was equal (i.e. the indifference point) was interpolated. Effects of nicotine (0.1-1.0 mg/kg, subcutaneous) on percent choice and indifference points were determined during the acute-testing phase and during the redetermination of effects of each dose after at least 30 sessions of repeated 1.0 mg/kg nicotine exposure. The Lewis rats had shorter indifference points (i.e. made fewer larger-reinforcer choices) compared with the Fischer 344 rats. Acute nicotine administration increased the mean larger-reinforcer choices at the 0.3 mg/kg dose in the Lewis rats and at the 1.0 mg/kg dose in the Fischer 344 rats. After repeated exposure to nicotine, indifference points returned to near-baseline (predrug) levels for both the strains. Strain differences were observed in the rates of delay discounting, and nicotine may decrease the impulsive choice acutely, but this effect does not seem to be long lasting.

Lack of GABAB receptors modifies behavioural and biochemical alterations induced by precipitated nicotine withdrawal

The nicotine (NIC) withdrawal syndrome is considered to be a major cause of the high relapse rate among individuals undergoing smoking cessation. The aim of the present study was to evaluate a possible role of GABAB receptors in NIC withdrawal, by comparing GABAB1 knockout mice and their wild-type littermates. We analysed the time course of the global withdrawal score, the anxiety-like effects, monoamine concentrations, the brain-derived neurotrophic factor (BDNF) expression, the corticosterone plasmatic levels and [(3)H]epibatidine binding sites during NIC withdrawal precipitated by mecamylamine, a nicotinic receptor antagonist (MEC). In NIC withdrawn wild-type mice, we observed a global withdrawal score, an anxiety-like effect in the elevated plus maze, a decrease of the striatal dopamine and 3,4-dihydroxyphenylacetic acid concentrations, an increase of corticosterone plasma levels, a reduction of BDNF expression in several brain areas and an increase of [(3)H]epibatidine binding sites in specific brain regions. Interestingly, the effects found in NIC withdrawn wild-type mice were absent in GABAB1 knockout mice, suggesting that GABAB1 subunit of the GABAB receptor is involved in the regulation of the behavioural and biochemical alterations induced by NIC withdrawal in mice. These results reveal an interaction between the GABAB receptors and the neurochemical systems through which NIC exerts its long-term effects.

Neurocognitive endophenotypes in schizophrenia: modulation by nicotinic receptor systems

Cigarette smoking is the leading preventable cause of death in the Western world, with a considerably higher prevalence observed in schizophrenia compared to the general population. Despite the negative health consequences of smoking heavily, it has been proposed that individuals with schizophrenia may maintain smoking behaviors to remediate symptoms associated with the disorder. Neurocognitive deficits are a core feature of schizophrenia and are present in approximately 80% of patients. Further, these deficits constitute an endophenotype of schizophrenia, as they are stable across disease phases, and are heritable. The neurocognitive deficits that are present in schizophrenia are especially debilitating, since they are associated with poor clinical and functional outcomes and community integration. Interestingly, these deficits may also constitute a vulnerability factor towards the initiation and maintenance of tobacco use. Contributing to the potential shared vulnerability between schizophrenia and tobacco dependence is a dysregulation of the nicotinic acetylcholine receptor (nAChR) system. Pre-clinical evidence has shown that nicotine affects several neurotransmitter systems, including dopamine (DA), glutamate, and γ-aminobutyric acid (GABA), and certain neuropsychological deficits associated with these neurotransmitters (reaction time, spatial working memory, sustained attention, and sensory gating) are improved after nicotine administration in patients with schizophrenia. These positive effects on neurocognition appear to be more pronounced in smokers with schizophrenia, and may be an important mechanism that explains the co-morbidity of schizophrenia and tobacco dependence.

Trait differences in response to chronic nicotine and nicotine withdrawal in rats

INTRODUCTION

Understanding an individual's vulnerability to drug addiction has important implications for the development of effective personal treatment plans. Although theories acknowledge impulsive behaviour as a key component of drug addiction, little is known about the influence of trait impulsivity on an individual's susceptibility to the effects of psychostimulants on impulsivity at critical phases of the addiction cycle.

METHODS

This study investigated the short and longer-term effects of chronic nicotine administration on impulsive choice in rats selected for high (HI) and low impulsivity (LI) on a delay discounting task. Rats prepared with subcutaneously osmotic mini-pumps received either nicotine (3.16 mg/kg/day [freebase]) or saline for 7 days. Performance was assessed during chronic treatment, early and late withdrawal, and in response to acute nicotine challenges following prolonged abstinence.

RESULTS

Chronic nicotine increased impulsive choice in LI but not HI animals. Spontaneous withdrawal was associated with a nicotine abstinence syndrome, the early stages of which were characterised by opposing effects on impulsive choice in HI and LI animals. A transient decrease in impulsivity was observed in HI animals whilst the LI group remained more impulsive for up to 1 week following drug termination. Following normalisation of behaviour, acute nicotine challenges (0.125, 0.25, 0.5 mg/kg, SC) markedly increased impulsive choice regardless of trait impulsivity and drug history.

CONCLUSION

The results indicate that only LI individuals are vulnerable to chronic drug- and withdrawal-induced impairments in self-control which may increase the likelihood of the transition to, and maintenance of, nicotine dependence.

Involvement of glutamatergic and GABAergic systems in nicotine dependence: Implications for novel pharmacotherapies for smoking cessation

Tobacco smoking continues to be a major global health hazard despite significant public awareness of its harmful consequences. Although several treatment options are currently available for smoking cessation, these medications are effective in only a small subset of smokers, and relapse rates continue to be high. Therefore, a better understanding of the neurobiological mechanisms that mediate tobacco dependence is essential for the development of effective smoking cessation medications. Nicotine is the primary psychoactive component of tobacco that drives the harmful tobacco smoking habit. Nicotine binds to nicotinic acetylcholine receptors (nAChRs) in the brain, resulting in the release of a wide range of neurotransmitters, including glutamate and γ-aminobutyric acid (GABA). This review article focuses on the role of the excitatory glutamate system and inhibitory GABA system in nicotine dependence. Accumulating evidence suggests that blockade of glutamatergic transmission or facilitation of GABAergic transmission attenuates the positive reinforcing and incentive motivational aspects of nicotine, inhibits the reward-enhancing and conditioned rewarding effects of nicotine, and blocks nicotine-seeking behavior. Chronic nicotine exposure produced long-term neuroadaptations that contribute to nicotine withdrawal, but the role of GABA and glutamate transmission in nicotine withdrawal is less understood. Overall, the findings presented in this review provide strong converging evidence for the potential effectiveness of glutamatergic and GABAergic medications in nicotine dependence. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Inhibition of monoamine oxidase isoforms modulates nicotine withdrawal syndrome in the rat

AIMS

There have been many reports of monoamine oxidase (MAO) inhibition by non-nicotine ingredients in tobacco smoke, persisting for days after smoking cessation. This study determined the effect of inhibiting MAO and its isoforms on nicotine withdrawal syndrome.

MAIN METHODS

Rats were rendered nicotine-dependent by seven days of subcutaneous (s.c.) 9 mg/kg/day infusion of nicotine bitartrate. Twenty-two hours after termination of infusion, they were observed over 20 min for somatically expressed nicotine withdrawal signs. Three hours before observation, rats were injected intraperitoneally (i.p.) with 4 mg/kg each of the MAO A antagonist clorgyline and the MAO B antagonist deprenyl, or with saline alone. A similar experiment was performed with non-dependent, saline-infused rats. Another experiment compared nicotine-dependent rats that received injections of either saline or 4 mg/kg clorgyline alone. A further experiment compared rats receiving either saline or 4 mg/kg deprenyl alone.

KEY FINDINGS

Combined treatment with both MAO inhibitors markedly and significantly exacerbated somatically expressed nicotine withdrawal signs in nicotine infused rats, while having no significant effects in saline-infused rats. Rats injected s.c. with 4 mg/kg clorgyline alone had significantly more withdrawal signs than saline-injected rats, while deprenyl-injected rats had significantly fewer signs than saline controls. Assays confirmed that clorgyline thoroughly reduced MAO A enzymatic activity and deprenyl thoroughly reduced MAO B activity.

SIGNIFICANCE

The results suggest that inhibition of MAO A may contribute to the intensity of withdrawal syndrome in smoking cessation.

Mecamylamine-precipitated nicotine withdrawal syndrome and its prevention with baclofen: an autoradiographic study of α4β2 nicotinic acetylcholine receptors in mice

A previous study from our laboratory showed that baclofen (BAC, GABAB receptor agonist) was able to prevent the behavioral expression of nicotine (NIC) withdrawal syndrome. To further investigate the mechanisms underlying this effect, we conducted this study, with the aims of analyzing α4β2 nicotinic receptor density during NIC withdrawal and, in case we found any changes, of determining whether they could be prevented by pretreatment with BAC. Swiss Webster albino mice received NIC (2.5 mg/kg, s.c.) 4 times daily, for 7 days. On the 8th day, NIC-treated mice received the nicotinic antagonist mecamylamine (MEC; 2 mg/kg, i.p.) 1 h after the last dose of NIC. A second group of NIC-treated mice received BAC (2 mg/kg, i.p.) prior to MEC administration. Thirty minutes after MEC, mice were sacrificed and brain autoradiography with [(3)H]epibatidine was carried out at five different anatomical levels. Autoradiographic mapping showed a significant increase of α4β2 nicotinic receptor labeling during NIC withdrawal in the nucleus accumbens shell (AcbSh), medial habenular nucleus (HbM), thalamic nuclei, dorsal lateral geniculate (DLG) nucleus, fasciculus retroflexus (fr), ventral tegmental area, interpeduncular nucleus and superior colliculus. BAC pretreatment prevented the increased α4β2 nicotinic receptor binding sites in the AcbSh, MHb, thalamic nuclei, DLG nucleus and fr. The present results suggest a relationship between BAC's preventive effect of the expression of NIC withdrawal signs, and its ability to restore the changes in α4β2 nicotinic receptor labeling, evidenced in specific brain areas in NIC withdrawn animals.

Animal models of nicotine exposure: relevance to second-hand smoking, electronic cigarette use, and compulsive smoking

Much evidence indicates that individuals use tobacco primarily to experience the psychopharmacological properties of nicotine and that a large proportion of smokers eventually become dependent on nicotine. In humans, nicotine acutely produces positive reinforcing effects, including mild euphoria, whereas a nicotine abstinence syndrome with both somatic and affective components is observed after chronic nicotine exposure. Animal models of nicotine self-administration and chronic exposure to nicotine have been critical in unveiling the neurobiological substrates that mediate the acute reinforcing effects of nicotine and emergence of a withdrawal syndrome during abstinence. However, important aspects of the transition from nicotine abuse to nicotine dependence, such as the emergence of increased motivation and compulsive nicotine intake following repeated exposure to the drug, have only recently begun to be modeled in animals. Thus, the neurobiological mechanisms that are involved in these important aspects of nicotine addiction remain largely unknown. In this review, we describe the different animal models available to date and discuss recent advances in animal models of nicotine exposure and nicotine dependence. This review demonstrates that novel animal models of nicotine vapor exposure and escalation of nicotine intake provide a unique opportunity to investigate the neurobiological effects of second-hand nicotine exposure, electronic cigarette use, and the mechanisms that underlie the transition from nicotine use to compulsive nicotine intake.

Stress response dysregulation and stress-induced analgesia in nicotine dependent men and women

Alterations in the stress response and endogenous pain regulation mechanisms may contribute directly and indirectly to maintenance of nicotine dependence and relapse. We examined the extent to which nicotine dependence alters endogenous pain regulatory systems, including the hypothalamic-pituitary-adrenocortical axis, cardiovascular activity, and stress-induced analgesia. Smokers and nonsmokers attended a laboratory session that included assessment of hormonal and cardiovascular responses to stress. Smokers smoked at their regular rate prior to the session. The hand cold pressor and heat thermal pain tests were completed twice, once after acute stress (public speaking and math tasks) and the other after rest. While smokers and nonsmokers exhibited significant hormonal and cardiovascular responses to stress, smokers exhibited blunted stress responses relative to nonsmokers. They also exhibited diminished stress-induced analgesia. Results demonstrate altered stress response and diminished stress-induced analgesia among chronic smokers, and suggest that these dysregulated physiological responding may contribute to altered endogenous pain regulation.

Acute behavioural responses to nicotine and nicotine withdrawal syndrome are modified in GABA(B1) knockout mice

Nicotine is the main active component of tobacco, and has both acute and chronic pharmacological effects that can contribute to its abuse potential in humans. The aim of the present study was to evaluate a possible role of GABA(B) receptors in acute and chronic responses to nicotine administration, by comparing GABA(B1) knockout mice and their wild-type littermates. In wild-type mice, acute nicotine administration (0.5, 1, 3 and 6 mg/kg, sc) dose-dependently decreased locomotor activity, and induced antinociceptive responses in the tail-immersion and hot-plate tests. In GABA(B1) knockout mice, the hypolocomotive effect was observed only with the highest dose of nicotine, and the antinociceptive responses in both tests were significantly reduced in GABA(B1) knockout mice compared to their wild-type littermate. Additionally, nicotine elicited anxiolytic- (0.05 mg/kg) and anxiogenic-like (0.8 mg/kg) responses in the elevated plus-maze test in wild-type mice, while selectively the anxiolytic-like effect was abolished in GABA(B1) knockout mice. We further investigated nicotine withdrawal in mice chronically treated with nicotine (25 mg/kg/day, sc). Mecamylamine (1 mg/kg, sc) precipitated several somatic signs of nicotine withdrawal in wild-type mice. However, signs of nicotine withdrawal were missing in GABA(B1) knockout mice. Finally, there was a decreased immunoreactivity of Fos-positive nuclei in the bed nucleus of the stria terminalis, basolateral amygdaloid nucleus and hippocampal dentate gyrus in abstinent wild-type but not in GABA(B1) knockout mice. These results reveal an interaction between the GABA(B) system and the neurochemical systems through which nicotine exerts its acute and long-term effects.

Adolescent rats are resistant to adaptations in excitatory and inhibitory mechanisms that modulate mesolimbic dopamine during nicotine withdrawal

Adolescent smokers report enhanced positive responses to tobacco and fewer negative effects of withdrawal from this drug than adults, and this is believed to propel higher tobacco use during adolescence. Differential dopaminergic responses to nicotine are thought to underlie these age-related effects, as adolescent rats experience lower withdrawal-related deficits in nucleus accumbens (NAcc) dopamine versus adults. This study examined whether age differences in NAcc dopamine during withdrawal are mediated by excitatory or inhibitory transmission in the ventral tegmental area (VTA) dopamine cell body region. In vivo microdialysis was used to monitor extracellular levels of glutamate and gamma-aminobutyric acid (GABA) in the VTA of adolescent and adult rats experiencing nicotine withdrawal. In adults, nicotine withdrawal produced decreases in VTA glutamate levels (44% decrease) and increases in VTA GABA levels (38% increase). In contrast, adolescents did not exhibit changes in either of these measures. Naïve controls of both ages did not display changes in NAcc dopamine, VTA glutamate, or VTA GABA following mecamylamine. These results indicate that adolescents display resistance to withdrawal-related neurochemical processes that inhibit mesolimbic dopamine function in adults experiencing nicotine withdrawal. Our findings provide a potential mechanism involving VTA amino acid neurotransmission that modulates age differences during withdrawal.

Ability of baclofen to prevent somatic manifestations and neurochemical changes during nicotine withdrawal

BACKGROUND

Nicotine (NIC), the major active component of tobacco, is critical in the maintenance of the smoking habit. The aims of the present study were to analyze the behavioural and neurochemical variations during NIC withdrawal syndrome in mice, and whether they are prevented with baclofen (BAC, GABA(B) receptor agonist).

METHODS

Swiss-Webster albino mice received NIC (2.5 mg/kg, s.c.) 4 times daily, for 7 consecutive days. On day 8 (the day of the experiment), NIC-treated mice received the nicotine antagonist mecamylamine (MEC, 2 mg/kg, i.p.) 1h after the last dose of NIC. A second group of dependent mice received BAC (2mg/kg, i.p.) before MEC-precipitated abstinence. The somatic signs were measured for 30 min. Dopamine (DA), serotonin (5-hydroxytryptamine; 5-HT) and its metabolites concentrations were determined by HPLC in the striatum, cortex and hippocampus.

RESULTS

The global score was greater in the abstinent group compared to the control group. Moreover, the global score time course showed a higher increase at 10 min compared to the global score at 5 min or 30 min after MEC-precipitated NIC withdrawal. In addition, the global score was attenuated by BAC. The DA and dihydroxyphenyl acetic acid (DOPAC) cortical levels decreased in the abstinent group, while BAC reestablished these levels 10 min after NIC withdrawal. Furthermore, DA and 5-HT striatal levels decreased during NIC withdrawal, and BAC reverted this decrease.

CONCLUSION

In conclusion, the prevention of NIC withdrawal signs by BAC could be related to changes in dopaminergic and serotonergic activity.

The alpha2 adrenergic receptor antagonist idazoxan, but not the serotonin-2A receptor antagonist M100907, partially attenuated reward deficits associated with nicotine, but not amphetamine, withdrawal in rats

Based on phenomenological similarities between anhedonia (reward deficits) associated with drug withdrawal and the negative symptoms of schizophrenia, we showed previously that the atypical antipsychotic clozapine attenuated reward deficits associated with psychostimulant withdrawal. Antagonism of alpha(2) adrenergic and 5-HT(2A) receptors may contribute to these effects of clozapine. We investigated here whether blockade of alpha(2) or 5-HT(2A) receptors by idazoxan and M100907, respectively, would reverse anhedonic aspects of psychostimulant withdrawal. Idazoxan treatment facilitated recovery from spontaneous nicotine, but not amphetamine, withdrawal by attenuating reward deficits and increase the number of somatic signs. Thus, alpha(2) adrenoceptor blockade may have beneficial effects against nicotine withdrawal and may be involved in the effects of clozapine previously observed. M100907 worsened the anhedonia associated with nicotine and amphetamine withdrawal, suggesting that monotherapy with M100907 may exacerbate the expression of the negative symptoms of schizophrenia or nicotine withdrawal symptoms in people, including schizophrenia patients, attempting to quit smoking.

Neuromodulation of decision-making in the addictive brain

Noninvasive brain stimulation of the dorsolateral prefrontal cortex with repetitive transcranial magnetic stimulation and transcranial direct current stimulation can modify decision-making behaviors in healthy subjects. The same type of noninvasive brain stimulation can suppress drug craving in substance user patients, who often display impaired decision-making behaviors. We discuss the implications of these studies for the cognitive neurosciences and their translational applications to the treatment of addictions. We propose a neurocognitive model that can account for our findings and suggests a promising therapeutic role of brain stimulation in the treatment of substance abuse and addictive behavior disorders.

The nicotinic acetylcholine receptor CHRNA5/A3/B4 gene cluster: dual role in nicotine addiction and lung cancer

More than 1 billion people around the world smoke, with 10 million cigarettes sold every minute. Cigarettes contain thousands of harmful chemicals including the psychoactive compound, nicotine. Nicotine addiction is initiated by the binding of nicotine to nicotinic acetylcholine receptors, ligand-gated cation channels activated by the endogenous neurotransmitter, acetylcholine. These receptors serve as prototypes for all ligand-gated ion channels and have been extensively studied in an attempt to elucidate their role in nicotine addiction. Many of these studies have focused on heteromeric nicotinic acetylcholine receptors containing α4 and β2 subunits and homomeric nicotinic acetylcholine receptors containing the α7 subunit, two of the most abundant subtypes expressed in the brain. Recently however, a series of linkage analyses, candidate-gene analyses and genome-wide association studies have brought attention to three other members of the nicotinic acetylcholine receptor family: the α5, α3 and β4 subunits. The genes encoding these subunits lie in a genomic cluster that contains variants associated with increased risk for several diseases including nicotine dependence and lung cancer. The underlying mechanisms for these associations have not yet been elucidated but decades of research on the nicotinic receptor gene family as well as emerging data provide insight on how these receptors may function in pathological states. Here, we review this body of work, focusing on the clustered nicotinic acetylcholine receptor genes and evaluating their role in nicotine addiction and lung cancer.

Tobacco addiction: a biochemical model of nicotine dependence

Nicotine is the main psychoactive substance present in tobacco, targeting in the CNS the nicotinic acetylcholine receptors (nAChR). The main effects of nicotine associated with smoking are nAChR upregulation, nAChR desensitization and modulation of the dopaminergic system. However, there is a lack of a comprehensive explanation of their roles that effectively makes clear how nicotine dependence might be established on those grounds. Receptor upregulation is an unusual effect for a drug of abuse, because theoretically this implies less need for drug consumption. Receptor upregulation and receptor desensitization are commonly viewed as opposite, homeostatic mechanisms. We here analyze the available information under a model in which both receptor upregulation and receptor desensitization are responsible for establishing a mechanism of nicotine dependence, consequently having an important role in starting and maintaining tobacco addiction. We propose that negative feedbacks on dopamine release regulated by alpha4beta2 nAChRs are disrupted by nicotine. nAChR desensitization is the disrupting mechanism, while nAChR upregulation is the reinforcing process of nicotine dependence, which eventually initiates tobacco addiction. A conclusion of the model is that drugs used for smoking cessation should inhibit preferentially alpha4beta2 nAChRs and to have a low or null ability to upregulate nAChRs, as this characteristic allows the smoker to achieve downregulation without abstinence symptoms. A relationship between this hypothesis and smoking and schizophrenia is also discussed.

Nicotine dependence: biology, behavior, and treatment

Despite great advances in the understanding and treatment of nicotine dependence, close to 21% of adults in the United States continue to smoke. Tobacco use is the single greatest cause of premature and preventable death in the United States. This article reviews the epidemiology, assessment, neurobiology, genetic etiology, and treatment of nicotine dependence. Enhanced understanding of these dimensions of nicotine dependence may help to advance progress toward lowering the prevalence rate of tobacco use in the U.S. and lowering the rate of tobacco-related morbidity and mortality.

Mechanisms underlying the comorbidity of tobacco use in mental health and addictive disorders

We discuss potential explanations for the high prevalence of tobacco use and tobacco dependence (TD) in people with mental health and addictive (MHA) disorders. The biopsychosocial basis for this comorbidity is presented, integrating evidence from epidemiologic and clinical studies. We also review evidence that suggests a shared vulnerability related to biological, genetic, and environmental factors may be the most parsimonious mechanism to explain the association between TD and MHA disorders. Finally, we review the examples of various MHA disorders that are associated with TD, and suggest avenues for new investigation that could aid in the development of rationale and more effective treatments for tobacco and MHA disorder comorbidities.

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.

The Neurobiology of Nicotine Addiction: Clinical and Public Policy Implications

Clinicians, social scientists, researchers, and policy makers appreciate the need to understand the neurobiology of nicotine addiction and how this information can lead to new treatments and provide support for public policy debates on parity and preventing adolescent tobacco use. In a “bench-to-bedside” manner, this review covers both clinical and basic science perspectives. Both the reward and sensitization-homeostasis theories of nicotine addiction are supported by new understanding of clinical issues of rapid tolerance, withdrawal, sensitization, and craving when examined by functional brain imaging, genetics, and basic science studies of nicotinic acetylcholine receptors. This review provides information to help shape public policy, fight stigma, and improve clinical treatment and research. The fight for parity in health care requires education about the neurobiological basis of addiction versus the stigmatized bad habit or simple socialization. Parity must support reimbursement for nicotine replacement medications or other FDA approved medications and psychosocial treatments.

Cellular events in nicotine addiction

In the 25 years since the observation that chronic exposure to nicotine could regulate the number and function of high affinity nicotine binding sites in the brain there has been a major effort to link alterations in nicotinic acetylcholine receptors (nAChRs) to nicotine-induced behaviors that drive the addiction to tobacco products. Here we review the proposed roles of various nAChR subtypes in the addiction process, with emphasis on how they are regulated by nicotine and the implications for understanding the cellular neurobiology of addiction to this drug.

Animal models and treatments for addiction and depression co-morbidity

The high rates of co-morbidity of drug addiction with depression may be attributable to shared neurobiology. Here, we discuss shared neurobiological substrates in drug withdrawal and depression, with an emphasis on changes in brain reward circuitry that may underlie anhedonia, a core symptom of depression and drug withdrawal. We explored experimentally whether clinical antidepressant medications or other treatments would reverse the anhedonia observed in rats undergoing spontaneous nicotine or amphetamine withdrawal, defined operationally as elevated brain reward thresholds. The co-administration of selective serotonin reuptake inhibitors with a serotonin-1A receptor antagonist, or the tricyclic antidepressant desipramine, or the atypical antidepressant bupropion ameliorated nicotine or amphetamine withdrawal in rats. Thus, increases in monoaminergic neurotransmission, or neuroadaptations induced by increased monoaminergic neurotransmission, ameliorated depression-like aspects of drug withdrawal. Further, chronic pretreatment with the atypical antipsychotic clozapine, that has some efficacy in the treatment of the depression-like symptoms of schizophrenia, attenuated nicotine and amphetamine withdrawal. Finally, a metabotropic glutamate 2/3 receptor antagonist reversed threshold elevations associated with nicotine withdrawal. The effects of these pharmacological manipulations are consistent with the altered neurobiology observed in drug withdrawal and depression. Thus, these data support the hypothesis of common substrates mediating the depressive symptoms of drug withdrawal and those seen in psychiatric patients. Accordingly, the anhedonic state associated with drug withdrawal can be used to study the neurobiology of anhedonia, and thus contribute to the identification of novel targets for the treatment of depression-like symptoms seen in various psychiatric and neurological disorders.

Effects of expectancy and abstinence on the neural response to smoking cues in cigarette smokers: an fMRI study

Cues associated with drug taking can trigger relapse, drug seeking, and craving in addicted individuals. Behavioral studies suggest that drug availability and withdrawal can affect the individual response to drug cues. Moreover, the importance of subjective craving in cue-induced relapse has been questioned and an alternative model put forward according to which drug cues trigger habitual drug-seeking behaviors independently of craving. We used functional magnetic resonance imaging to compare the brain response to smoking and control videotapes in 20 healthy smokers, while varying their expectancy to smoke and abstinence levels. The neural response to cigarette cues was strongly modulated by expectancy and, to a lesser extent, abstinence. In people expecting to smoke immediately after the scan, smoking cues activated brain areas implicated in arousal, attention, and cognitive control. However, when subjects knew they would not be allowed to smoke for 4 h, there was almost no brain activation in response to smoking cues, despite equivalent reported levels of craving. In the dorsolateral prefrontal cortex, the neural response was a function of both craving and expectancy. Thalamo-cingulate connectivity, thought to be an index of arousal, was greater during expectancy than nonexpectancy. Our findings confirm the importance of expectancy in the neural response to drug cues, and lend support to the theory that these cues act on brain areas involved in arousal and attention.

Genetic dissociation of two behaviors associated with nicotine addiction: beta-2 containing nicotinic receptors are involved in nicotine reinforcement but not in withdrawal syndrome

RATIONALE

Nicotine addiction is characterized by two distinct behaviors, chronic compulsive self-administration and the induction of a withdrawal syndrome upon cessation of nicotine consumption.

OBJECTIVE

To examine if these two processes rely on beta2-containing nicotinic receptors--beta2*nAChRs--we analyzed the behavior of mice lacking these receptors in the two situations.

RESULTS

First, we showed that, in contrast to wild-type (WT) mice, beta2-knockout (beta2-/-) mice exhibit no intra-ventral tegmental area (VTA) nicotine self-administration, whereas their ability to self-administer morphine is intact. However, beta2-/- mice showed some sensitivity to locomotor effects of nicotine, implying an effect of the drug on other nicotinic subtypes. Then, we observed that beta2-/- mice exhibited a normal nicotine withdrawal syndrome, i.e., increased levels of rearing and jumping upon precipitated withdrawal. Thus, the beta2*nAChRs are not involved in the behaviors induced by cessation of nicotine consumption.

CONCLUSION

Taken together, the present data demonstrated a genetic dissociation of two distinct behavioral patterns associated with nicotine addiction. They further suggested that independent molecular mechanisms underlie these two aspects, offering the possibility of controlling them separately.

Bupropion attenuates nicotine abstinence syndrome in the rat

RATIONALE

Bupropion reduces discomfort and craving associated with smoking cessation. This study determined whether a rat model of nicotine dependence could detect such nicotine abstinence-alleviating effects.

OBJECTIVES

Experiments determined whether the abstinence-alleviating effects of bupropion were detectable by (1) behavioral abstinence signs precipitated by the nicotinic antagonist mecamylamine, (2) place aversion conditioned to mecamylamine-precipitated nicotine abstinence, and (3) spontaneous behavioral abstinence signs after abrupt nicotine withdrawal.

METHODS

In experiments 1 and 2, nicotine-dependent rats were coinfused for 7 days with 3.15 mg/kg/day nicotine and 20 mg/kg/day bupropion or with nicotine alone. They were then challenged with 1 mg/kg mecamylamine and observed for behavioral abstinence signs (experiment 1) or place aversion conditioned to precipitated abstinence (experiment 2). In experiment 3, rats were nicotine-infused for 7 days as above. A day after termination of nicotine infusion, rats were observed for spontaneous nicotine abstinence signs before and after injection with saline or bupropion.

RESULTS

In experiment 1, rats coinfused with nicotine and bupropion had significantly fewer mecamylamine-precipitated abstinence signs than rats infused with nicotine alone but similar numbers to rats infused with saline alone. In experiment 2, bupropion pretreatment significantly reduced the aversiveness of mecamylamine-precipitated nicotine abstinence. In experiment 3, a single bupropion injection dose-dependently alleviated spontaneous nicotine abstinence syndrome.

CONCLUSIONS

These results suggest that these rat models of nicotine dependence and abstinence syndrome may be useful in detecting nicotine abstinence-alleviating effects of potential medications for smoking cessation. The effects of acute bupropion administration raise interesting questions regarding bupropion's mechanism of action.

Brain activity in cigarette smokers performing a working memory task: effect of smoking abstinence

BACKGROUND

When nicotine-dependent human subjects abstain from cigarette smoking, they exhibit deficits in working memory. An understanding of the neural substrates of such impairments may help to understand how nicotine affects cognition. Our aim, therefore, was to identify abnormalities in the circuitry that mediates working memory in nicotine-dependent subjects after they initiate abstinence from smoking.

METHODS

We used blood oxygen level-dependent (BOLD) functional magnetic resonance imaging (fMRI) to study eight smokers while they performed a letter version of the N-Back working memory task under satiety (< or = 1.5 hours abstinence) and abstinence (> or = 14 hours abstinence) conditions.

RESULTS

Task-related activity in the left dorsal lateral prefrontal cortex (DLPFC) showed a significant interaction between test session (satiety, abstinence) and task load (1-back, 2-back, and 3-back). This interaction reflected the fact that task-related activity in the satiety condition was relatively low during performance of the 1-back task but greater at the more difficult task levels, whereas task-related activity in the abstinence condition was relatively high at the 1-back level and did not increase at the more difficult task levels.

CONCLUSIONS

We conclude that neural processing related to working memory in the left DLPFC is less efficient during acute abstinence from smoking than at smoking satiety.

Nicotinic receptor mechanisms and cognition in normal states and neuropsychiatric disorders

Cigarette smoking rates in the American population are approximately 23%, whereas rates of smoking in clinical and population studies of individuals with neuropsychiatric disorders are typically two- to four-fold higher. Studies conducted in a variety of neuropsychiatric populations [e.g. attention-deficit hyperactivity disorder (ADHD), Alzheimer's disease, schizophrenia] have collectively suggested that nicotine may be efficacious in remediating selected cognitive deficits associated with these disorders, thus providing a framework for understanding the specific vulnerability of these patients to smoking initiation and maintenance. However, the specific gain in cognitive performance produced by nicotine administration in healthy subjects with normal cognitive function is less clear. This article reviews our current understanding of central nicotinic acetylcholine receptor (nAChRs) systems in normal and neuropsychiatric disease states and, specifically, their role with respect to cognitive dysfunction and clinical symptoms in several specific neuropsychiatric populations, including ADHD, Alzheimer's disease, Parkinson's disease, Tourette's Disorder, schizophrenia and affective disorders. The potential benefits of nicotinic agents for therapeutic use in neuropsychiatric disorders is discussed, as well as directions for further research in this area.

Co-morbidity of smoking in patients with psychiatric and substance use disorders

This article reviews cigarette smoking in patients with psychiatric disorders (PD) and substance use disorders (SUD). Rates of smoking are approximately 23% in the U.S. population but approximately two- to four-fold higher in patients with PD and SUD. Many remaining smokers have had repeated smoking cessation failures, possibly due to the presence of co-morbid PD and SUDs. There is modest, evidence-based support for effective treatment interventions for nicotine addiction in PD and SUD. Further research is needed to increase our understanding of nicotine addiction in PD and SUD and develop more effective treatment interventions.

Nicotinic receptor mechanisms and cognition in normal states and neuropsychiatric disorders

Cigarette smoking rates in the American population are approximately 23%, whereas rates of smoking in clinical and population studies of individuals with neuropsychiatric disorders are typically two- to four-fold higher. Studies conducted in a variety of neuropsychiatric populations [e.g. attention-deficit hyperactivity disorder (ADHD), Alzheimer's disease, schizophrenia] have collectively suggested that nicotine may be efficacious in remediating selected cognitive deficits associated with these disorders, thus providing a framework for understanding the specific vulnerability of these patients to smoking initiation and maintenance. However, the specific gain in cognitive performance produced by nicotine administration in healthy subjects with normal cognitive function is less clear. This article reviews our current understanding of central nicotinic acetylcholine receptor (nAChRs) systems in normal and neuropsychiatric disease states and, specifically, their role with respect to cognitive dysfunction and clinical symptoms in several specific neuropsychiatric populations, including ADHD, Alzheimer's disease, Parkinson's disease, Tourette's Disorder, schizophrenia and affective disorders. The potential benefits of nicotinic agents for therapeutic use in neuropsychiatric disorders is discussed, as well as directions for further research in this area.

Decrease of ventral tegmental area dopamine neuronal activity in nicotine withdrawal rats

The present study was designed to examine the ventral tegmental area (VTA) dopamine neuronal activity in nicotine withdrawal rats by means of in vivo single-unit extracellular recordings. Animals were treated with nicotine base (6 mg/kg/day, s.c.) four times daily for 12 days. One day after the last nicotine administration, the firing rates of the VTA dopamine neurons were found to be significantly decreased. Following 2, 3, 5 and 10 days of nicotine withdrawal, however, the firing rates returned to the control levels. The number of spontaneously active dopamine cells was not altered at any time points. The results indicate that the VTA dopamine neuronal activity is reduced during the first day of nicotine withdrawal.

Neuroendocrinological and neuropsychological correlates of dopaminergic function in nicotine dependence

RATIONALE

There is multiple evidence that nicotine--as with ethanol and other drugs of abuse--stimulates dopamine release in the ventral striatum as a central part of the brain reward circuits. Chronic nicotine exposure leads to changes in these dopaminergic reward circuits. During nicotine withdrawal, an impaired dopaminergic function has been reported. On the behavioral level, this seems to result in motivational disturbances in abstaining smokers.

OBJECTIVES

To investigate the impact of smoking on dopaminergic function in humans both on a neuroendocrinological and on a neuropsychological level.

METHODS

Thirty-seven healthy smokers were assessed whilst smoking (test 1) and after abstaining overnight for 12 h (test 2). A control group of 18 non-smokers was also examined twice. Severity of nicotine dependence, incentive motivation, digit span and verbal fluency were assessed. The sensitivity of central dopamine (DA) D2 receptors was assessed with the apomorphine-induced growth hormone (GH) secretion.

RESULTS

ANOVA revealed that GH response was significantly lower in smokers than in non-smokers (P=0.04). The GH response was significantly inversely correlated with severity of nicotine dependence (r=-0.39). Neuropsychological performance was not influenced by smoking status. After overnight abstinence from nicotine GH response, digit span and verbal fluency were not affected, whereas incentive motivation was significantly impaired in smokers (P=0.04).

CONCLUSIONS

Smoking is significantly associated with a reduced sensitivity of central DA D2 receptors. This alteration of dopaminergic sensitivity is stable even after 12 h of abstinence from nicotine. Therefore, the hypothesis that the motivational impairment during withdrawal from nicotine is associated with an altered sensitivity of central DA D2 receptors cannot be supported.

Cognitive and psychological correlates of smoking abstinence, and predictors of successful cessation

The neural circuitry implicated in addictive drug use, which appears to be down-regulated in early abstinence, corresponds closely with brain reward pathways. A literature review suggests that responses to incentive stimuli and the ability to inhibit reflexive responses, both of which have been associated with normal functioning in these pathways, might be weakened during acute abstinence from chronic drug use. In an ongoing study, 82 smokers, abstinent overnight before two separate testing occasions, have been assessed after administration of nicotine and placebo lozenges (order of sessions counterbalanced). Nicotine administration is associated with a significant reduction in anhedonia, a near-significant increase in response to financial incentive, enhanced ability to inhibit reflexive eye movements, and increased attentional bias to words with appetitive significance. Fifty-nine participants then initiated a quit attempt and 19 reported relapsing within 7 days. Comparing their performance in the two prequit lozenge assessment sessions, relapsers showed a stronger effect of nicotine on enhancing their ability to inhibit reflexive eye movements and a near-significant trend towards greater nicotine-induced increases in attentional bias toward appetitive words.

Imaging localised dynamic changes in the nucleus accumbens following nicotine withdrawal in rats

This study utilises pharmacological functional magnetic resonance imaging (fMRI) to examine the neurobiological mechanisms through which nicotine produces dependence. Using an established regime to induce physical dependence to nicotine in rats (osmotic minipumps delivering 3.16 mg/kg/day nicotine for 7 days SC), animals were subsequently anaesthetised under urethane and positioned in a stereotaxic frame to allow collection of gradient echo whole brain images with a 4.7-T MRI spectrometer. Rats were initially scanned for 34 min (40 baseline image volumes, 1 volume per 51 s) then challenged with mecamylamine (1.0 mg/kg SC) or saline (1 ml/kg) and scanned for a further 68 min (80 image volumes). Mecamylamine precipitated highly significant positive changes in fMRI blood oxygen level dependent (BOLD) contrast that were predominantly localised to the NAc of nicotine-dependent rats. Saline-treated rats challenged with the same dose of mecamylamine exhibited similar but smaller increases in BOLD contrast although such changes were less defined around the NAc. Precipitated withdrawal also elicited statistically significant negative BOLD contrast changes in widespread cortical regions. These findings are consistent with previous neurochemical reports on decreases in dopamine in the NAc during nicotine withdrawal. This fMRI study further highlights the potential and power to image the neurobiological events during nicotine dependence.

Long‐term, low‐level adolescent nicotine exposure produces dose‐dependent changes in cocaine sensitivity and reward in adult mice

Cigarette smoking by adolescents is a strong predictor of future drug use, abuse, and dependence. While this "gateway drug effect" is assumed to be related to psychosocial factors, data from our laboratory suggests that adolescent nicotine use may permanently disrupt reward systems through changes in dopamine receptor function. Behavioral pharmacological methods known to be indirectly (motor activity) and directly (conditioned-place-preference) related to drug reinforcement were used to examine changes in cocaine sensitivity. Testing was performed on adult mice that were exposed to nicotine (0.3, 1.0, and 3.0 mg/kg, SC, M-F, b.i.d.) or saline during adolescence (postnatal days 25-57). Prior to testing, subjects had a 28 day drug-free, time-off period. After acclimation to the testing apparatus, the locomotor effects (30 min, 30 cm traveled) of cocaine (5, 10, and 20 mg/kg, IP) were measured daily; cocaine tests were preceded and followed by saline control tests. Following the acute dose-response curve, mice received saline followed by 5 days of 20.0 mg/kg cocaine. Thereafter, mice underwent condition-place-preference testing. A pre-test was performed to determine compartment preference (i.e., no injection, 20 min test). Cocaine (10 mg/kg, IP) was paired with the subjects non-preferred side and saline with the other. Conditioning sessions were conducted for 8 days with the order of drug/saline injections counter-balanced across subjects. A drug-free, post-test occurred on the day following the final conditioning session. A dose-dependent relationship between adolescent nicotine exposure and cocaine reward was noted in the adult mice across both test conditions. Subjects exposed to nicotine showed an increased response to cocaine's motor activating effects and a decreased response to cocaine's rewarding effects. A follow-up study was undertaken to evaluate dopamine D1, D2, and D3 receptor function in adult mice exposed to the highest dose of nicotine from the first study. While both interesting and revealing, the results of motor activity tests with dopamine agonist only approached significance. Further research will be required to more fully examine the mechanism of action for the observed changes in cocaine reward. In summary, this is the first study to demonstrate a dose-response relationship between adolescent nicotine exposure and changes in cocaine reward and sensitivity during adulthood.

Measuring the emergence of tobacco dependence: the contribution of negative reinforcement models

This review of negative reinforcement models of drug dependence is part of a series that takes the position that a complete understanding of current concepts of dependence will facilitate the development of reliable and valid measures of the emergence of tobacco dependence. Other reviews within the series consider models that emphasize positive reinforcement and social learning/cognitive models. This review summarizes negative reinforcement in general and then presents four current negative reinforcement models that emphasize withdrawal, classical conditioning, self-medication and opponent-processes. For each model, the paper outlines central aspects of dependence, conceptualization of dependence development and influences that the model might have on current and future measures of dependence. Understanding how drug dependence develops will be an important part of future successful tobacco dependence measurement, prevention and treatment strategies.

Neuroadaptive changes in the mesoaccumbens dopamine system after chronic nicotine self-administration: A microdialysis study

There is little evidence to date to indicate if mesoaccumbens dopamine function at the neurochemical level is altered during early abstinence from chronic i.v. nicotine self-administration. Thus, a quantitative microdialysis (no-net-flux) approach was used to measure basal extracellular concentrations and extraction fractions of dopamine in the nucleus accumbens (ACB) of rats that self-administered nicotine i.v. for 25 days, as well as in rats serving as yoked comparison groups (yoked nicotine and yoked saline). After 24-48 h of the final self-administration session, there was a significant reduction in basal extracellular dopamine levels in the ACB of the self-administration group compared with the yoked saline group (1.35+/-0.15 nM versus 3.70+/-0.28 nM). The basal extracellular dopamine levels in the yoked nicotine group (1.46+/-0.20 nM) were not significantly different compared with the nicotine self-administration group. The in vivo extraction fraction of dopamine, an indirect measure of dopamine uptake, was significantly increased in the nicotine self-administration (86%) and yoked nicotine (91%) groups compared with the yoked saline group (77%). In addition, a marked reduction in the elevation of extracellular dopamine levels in the ACB occurred after a nicotine challenge as measured by conventional microdialysis in the self-administration (112% of basal) and yoked nicotine (121% of basal) groups as compared with a yoked saline (154% of basal) group. The reduced basal ACB dopamine levels in the nicotine groups during early abstinence appears to be due to increased clearance, suggesting increased dopamine uptake is occurring as a result of the chronic nicotine treatment. The reduced elevation of extracellular dopamine levels in the ACB upon nicotine challenge suggests a functional desensitization or downregulation phenomenon involving acetylcholine receptors (nicotinic nAChRs). Overall, these results provide clear evidence for a neuroadaptive change that alters dopamine transmission in the ACB during abstinence from chronic i.v. nicotine exposure.

Nicotine lowers the body-weight set-point in male rats

Set-point of body weight was assessed by the behavioral method of the weight threshold to hoard food. Intra-peritoneal injection of nicotine (0.02-0.05 mg/kg) significantly lowered the rats' body weight set-point by 8.5+/-2.7 and 19.2+/-2.6 g (p< or =0.05). These results suggest that the administration of low-dose nicotine lowers the body-weight set-point and may be important in predicting the anticipated gain in weight frequently reported upon withdrawal.

Chronic exposure of nicotine modulates the expressions of the cerebellar glial glutamate transporters in rats

Rats were given nicotine (25 ppm) in their drinking water at the start of their mating period in order to study the expressions of glutamate transporter subtypes in cerebellar astrocytes following the chronic exposure of nicotine after mating. After the offspring were delivered, each group was divided into two subgroups. One group, the control group, was given distilled water only and the other group, the experimental group, was given distilled water containing nicotine. The cerebellar astrocytes were prepared from 7 day-old pups at each group. Ten days after the cells were cultured, the expression of the glutamate transporter subtypes (GLAST and GLT-1) was determined using immunochemistry and immunoblotting. During the continuous treatments, the developmental expression patterns of the GLAST and GLT-1 in the cerebellum were also determined from 2, 4 and 8 week-old rats. The expression levels of GLAST in cultured astrocytes of both the pre- or post-natally exposed groups were higher than those of the control group. However, these expression levels of the continuously exposed group were lower than those of the control group. Compared to those of the control group, the GLT-1 expression levels of all the nicotine-treated groups were higher, particularly in the continuously treated group. According to the results from the immochemistry procedure, the cerebellar GLAST and GLT-1 expression levels of all nicotine-treated groups were lower than those of the control group at each age. However, the immunoblotting procedure showed that the cerebellar GLT-1 expression levels of all the nicotine-treated groups were higher than those of the control group, except for the rats that were continuously exposed for 8 weeks using immunoblotting. These results suggest that the expression of the glial GLAST and GLT-1 are altered differently depending on the initial exposure time and the particular period of nicotine exposure. In addition, nicotine exposure during gestation has persistent effects on glial cells.