The effects of acute and repeated nicotine treatment on nucleus accumbens dopamine and locomotor activity

Mechanisms involved in systemic nicotine-induced glutamatergic synaptic plasticity on dopamine neurons in the ventral tegmental area

Systemic exposure to nicotine induces glutamatergic synaptic plasticity on dopamine (DA) neurons in the ventral tegmental area (VTA), but mechanisms are largely unknown. Here, we report that single, systemic exposure in rats to nicotine (0.17 mg/kg free base) increases the ratio of DA neuronal currents mediated by AMPA relative to NMDA receptors (AMPA/NMDA ratio) assessed 24 h later, based on slice-patch recording. The AMPA/NMDA ratio increase is evident within 1 h and lasts for at least 72 h after nicotine exposure (and up to 8 d after repeated nicotine administration). This effect cannot be prevented by systemic injection of either α7-nAChR (nicotinic ACh receptor)-selective [methyllycaconitine (MLA)] or β2*-nAChR-selective [mecamylamine (MEC)] antagonists but is prevented by coinjection of MLA and MEC. In either nAChR α7 or β2 subunit knock-out mice, systemic exposure to nicotine still increases the AMPA/NMDA ratio. Preinjection in rats of a NMDA receptor antagonist MK-801((+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate), but neither DA receptor antagonists [SCH-23390 (R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine) plus haloperidol] nor a calcineurin inhibitor (cyclosporine), prevents the nicotine-induced increase in AMPA/NMDA ratio. After systemic exposure to nicotine, glutamatergic (but not GABAergic) transmission onto rat VTA DA neuronal inputs is enhanced. Correspondingly, DA neuronal firing measured 24 h after nicotine exposure using extracellular single-unit recording in vivo is significantly faster, and there is conversion of silent to active DA neurons. Collectively, these findings demonstrate that systemic nicotine acting via either α7- or β2*-nAChRs increases presynaptic and postsynaptic glutamatergic function, and consequently initiates glutamatergic synaptic plasticity, which may be an important, early neuronal adaptation in nicotine reward and reinforcement.

The sensitizing effect of acute nicotine on amphetamine-stimulated behavior and dopamine efflux requires activation of β2 subunit-containing nicotinic acetylcholine receptors and glutamate N-methyl-D-aspartate receptors

Nicotine has been demonstrated to enhance the subsequent use of illicit drugs in animals and humans. We previously demonstrated in female, Holtzman rats that one low dose of nicotine will potentiate locomotor activity and dopamine (DA) efflux in response to a subsequent low dose of d-amphetamine (AMPH) given 1-4 h later. In the present study, we show this also occurs in male rats and characterize the receptors required for the rapid sensitizing effect of nicotine on AMPH-stimulated locomotor behavior and AMPH-induced DA efflux. Pretreatment of male, Holtzman rats with a low dose (0.1 mg/kg, i.p.) of nicotine 2-4 h before a challenge with AMPH (0.32 mg/kg, i.p.) enhanced locomotor behavior as compared to saline pretreatment. Dihydro-β-erythroidine (DHβE), a relatively selective antagonist at β2 subunit-containing (β2∗) nicotinic acetylcholine receptors (nAChR), but not methyllycaconitine (MLA), a relatively selective antagonist at α7 nAChRs, blocked the sensitizing effect of nicotine on AMPH-stimulated locomotor activity. Pretreatment with varenicline, a partial agonist selective for β2∗ nAChRs, blocked the sensitizing effect of nicotine on AMPH-stimulated locomotor behavior. Nicotine pretreatment sensitized AMPH-induced DA overflow in slices from ventral (nucleus accumbens, NAc), but not dorsal striatum as compared to saline-pretreated rats. Nicotine sensitization of the DA overflow was blocked by DHβE. Pretreatment with the glutamate N-methyl-D-aspartate (NMDA) receptor antagonist (+)-MK-801 (0.1 mg/kg, s.c.) 30 min before nicotine blocked sensitization of both locomotion and DA overflow in response to AMPH challenge. These results demonstrate that activation of the β2∗ nAChRs and NMDA receptors are required for the rapid sensitizing effect of nicotine on AMPH actions. This article is part of a Special Issue entitled 'Trends in neuropharmacology: in memory of Erminio Costa'.

A second-order schedule of food reinforcement in rats to examine the role of CB1 receptors in the reinforcement-enhancing effects of nicotine

Nicotine is believed to enhance the motivational value of reinforcers. Although endogenous cannabinoids acting on CB1 receptors have been implicated in the motivational effects of nicotine, their role in the 'reinforcement-enhancing' properties of nicotine is unknown. This study compared the effect of acute and chronic non-contingent nicotine administration on responding for an unconditioned reinforcing stimulus (UCS) and a visual conditioned stimulus (CS) and the role of CB1 receptors was examined. Male hooded Lister rats were trained on a second-order schedule [FI 15' (FR5: S)] under which presentation of the CS (5s/5Hz light oscillation) was intermittently reinforced by the UCS (food). The rats were treated with daily saline or nicotine (0.4 mg/kg, subcutaneous [s.c.]) throughout the study. The effect of the acute nicotine challenge (0.05, 0.1 and 0.2 mg/kg, s.c.) and the CB1 receptor antagonist AM251 (0.1, 0.3 and 1 mg/kg, intraperitoneal [i.p.]) on responding for the CS and/or UCS was examined. The acute nicotine challenge increased responding for both the UCS and CS in the rats chronically treated with nicotine, an effect which was less robust in the nicotine-naive rats. AM251 significantly reduced responding for the UCS and CS, and an interaction with the nicotine challenge was found. These data support and extend the hypothesis that nicotine can enhance the motivational value of reinforcing stimuli and suggest the increases in responding produced by nicotine involve CB1 receptors. Furthermore, this study highlights the utility of second-order schedules of reinforcement for investigation of the neural circuits underlying the reinforcement-enhancing effects of nicotine.

Age-related differences in amphetamine sensitization: effects of prior drug or stress history on stimulant sensitization in juvenile and adult rats

Repeated intermittent exposure to stimulants progressively increases a drug's effect, with stressors capable of producing cross-sensitization to stimulants. Studies examining such sensitization during development are few, however, with results mixed. In Experiment 1, juvenile (P22) and adult (P64) female Sprague-Dawley rats were administered (daily for 4days) 1.5mg/kg or 3.0mg/kg amphetamine (1.5A and 3.0A groups), or saline (SAL group). In a second experiment, rats were exposed to either repeated restraint (60min/day for 4days; RS group) or were left non-manipulated in the home cage (NM group). Animals from both experiments were then challenged with 1.5mg/kg of amphetamine and sensitization assessed via locomotion and stereotypy after a 2-day and 3-wk washout period. When compared to SAL animals, 3.0A juveniles and adults exhibited evidence of locomotor sensitization 2days post-drug exposure, but this sensitization did not persist to the 3-week challenge. Compared to NM animals, RS animals showed stress-induced locomotor sensitization both 2days and 3weeks post-stress exposure, regardless of age. These results demonstrate that repeated drug/stress exposures prior to stimulant challenge are sufficient to induce behavioral sensitization among both juveniles and adults, with these effects particularly long-lasting following repeated stressor exposure.

Progressive and lasting amplification of accumbal nicotine-seeking neural signals

Although neuroadaptations in the nucleus accumbens (NAc) are thought to contribute to nicotine addiction, little is known about the chronic effects of nicotine on NAc neuronal activity. In the present experiment, rats were exposed to a 23 d period of nicotine self-administration (SA), a 30 d abstinence period, and a 7 d period of reexposure to SA. Chronic electrophysiological procedures were used to record the activity of individual NAc neurons on the 3rd and 23rd days of initial SA and on the 1st, 3rd, and 7th days of reexposure. Between-session comparisons showed that NAc neurons exhibit two patterns of plasticity under the present experimental conditions. First, phasic-increase firing patterns time-locked to the nicotine-reinforced lever press do not change during initial SA, but then show increases in prevalence and amplitude after abstinence, which persist during reexposure. Second, for neurons that show no phasic response time-locked to the nicotine-reinforced lever press, average baseline and SA firing rates decrease during initial SA, return to normal during abstinence, and decrease again during reexposure. As a combined consequence of the two types of neurophysiological plasticity, average firing rate of NAc neurons at the time of nicotine-directed behavior undergoes a progressive and persistent net amplification, across the successive stages of SA, abstinence, and reexposure. This net increase in NAc firing at the time of nicotine-directed behavior occurs in association with an increase in animals' motivation to seek nicotine. The adaptations that occur in nicotine-exposed animals do not occur in animals exposed to sucrose. The NAc neurophysiological plasticity potentially contributes to compulsive tobacco use.

Actions of octocoral and tobacco cembranoids on nicotinic receptors

Nicotinic acetylcholine receptors (AChRs) are pentameric proteins that form agonist-gated cation channels through the plasma membrane. AChR agonists and antagonists are potential candidates for the treatment of neurodegenerative diseases. Cembranoids are naturally occurring diterpenoids that contain a 14-carbon ring. These diterpenoids interact with AChRs in complex ways: as irreversible inhibitors at the agonist sites, as noncompetitive inhibitors, or as positive modulators, but no cembranoid was ever shown to have agonistic activity on AChRs. The cembranoid eupalmerin acetate displays positive modulation of agonist-induced currents in the muscle-type AChR and in the related gamma-aminobutyric acid (GABA) type A receptor. Moreover, cembranoids display important biological effects, many of them mediated by nicotinic receptors. Cembranoids from tobacco are neuroprotective through a nicotinic anti-apoptotic mechanism preventing excitotoxic neuronal death which in part could result from anti-inflammatory properties of cembranoids. Moreover, tobacco cembranoids also have anti-inflammatory properties which could enhance their neuroprotective properties. Cembranoids from tobacco affect nicotine-related behavior: they increase the transient initial ataxia caused by first nicotine injection into naive rats and inhibit the expression of locomotor sensitization to repeated injections of nicotine. In addition, cembranoids are known to act as anti-tumor compounds. In conclusion, cembranoids provide a promising source of lead drugs for many clinical areas, including neuroprotection, smoking-cessation, and anti-cancer therapies.

Blunted opiate modulation of prolactin response in smoking men and women

Endogenous opioids are integral in modulating drug reward, but it is believed that these may act through several mechanisms including hypothalamic-pituitary-adrenocortical (HPA) and dopamine pathways. This study was developed to examine how nicotine dependence alters endogenous opioid regulation of prolactin response, a peripheral marker of dopaminergic activity. Smokers and nonsmokers completed two sessions during which placebo or 50mg of naltrexone was administered, using a double-blind, counterbalanced design. Blood samples and mood measures were obtained during a resting absorption period, after exposure to two noxious stimuli (cold pressor and thermal pain), and during an extended recovery period. Opioid blockade increased prolactin response, indicating an inhibitory effect of the endogenous opioid system on prolactin, possibly mediated by reduced stimulatory effects of dopamine on this hormone. These responses were attenuated in smokers relative to nonsmokers. There was also a gender disparity in prolactin response, with women showing a stronger response to endogenous opioid modification than men regardless of smoking status. The attenuated effects of opioid blockade may reflect dysregulated opiodergic and dopaminergic effects. Results extend previous reports showing blunted opioid regulation of the HPA response in dependent smokers.

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

RATIONALE

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

OBJECTIVE

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

RESULTS

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

CONCLUSION

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

Nicotinic receptor-based therapeutics and candidates for smoking cessation

Tobacco dependence is the most preventable cause of death and is a chronic, relapsing disorder in which compulsive tobacco use persists despite known negative health consequences. All currently available cessation agents (nicotine, varenicline and bupropion) have limited efficacy and are associated with high relapse rates, revealing a need for more efficacious, alternative pharmacotherapies. The major alkaloid in tobacco, nicotine, activates nicotinic receptors (nAChRs) which increase brain extracellular dopamine producing nicotine reward leading to addiction. nAChRs are located primarily presynaptically and modulate synaptic activity by regulating neurotransmitter release. Subtype-selective nAChR antagonists that block reward-relevant mesocorticolimbic and nigrostriatal dopamine release induced by nicotine may offer advantages over current therapies. An innovative approach is to provide pharmacotherapies which are antagonists at nAChR subtypes mediating nicotine evoked dopamine release. In addition, providing multiple medications with a wider array of targets and mechanisms should provide more treatment options for individuals who are not responsive to the currently available pharmacotherapies. This review summarizes the currently available smoking cessation therapies and discusses emerging potential therapeutic approaches employing pharmacological agents which act as antagonists at nicotinic receptors.

Neurotransmission-related genetic polymorphisms, negative affectivity traits, and gender predict tobacco abstinence symptoms across 44 days with and without nicotine patch

Genetic and personality trait moderators of tobacco abstinence-symptom trajectories were assessed in a highly controlled study. Based on evidence suggesting their importance in stress reactivity and smoking, moderators studied were serotonin transporter gene (5-HTTLPR) and dopamine D2 receptor gene (DRD2) polymorphisms and personality traits related to negative affect (NA). Smokers were randomly assigned to quit smoking with nicotine or placebo patches. Financial incentives resulted in 80% verified abstinence across the 44-day study. Individuals with 1 or 2 short alleles of 5-HTTLPR (S carriers) experienced larger increases in NA symptoms than did those without a short allele. Nicotine replacement therapy (NRT) alleviated anxiety only in S carriers. NRT reduced NA to a greater extent in DRD2 A1 carriers than in A2A2 individuals during the 1st 2 weeks of treatment (when on the 21-mg patch); however, A1 carriers experienced a renewal of NA symptoms when switched to the 7-mg patch and when off the patch, while A2A2 individuals continued to benefit from NRT. The results suggest that the effects of genotype and treatment may vary across different durations of abstinence, treatment doses, and genotypes.

The role of alpha6-containing nicotinic acetylcholine receptors in nicotine reward and withdrawal

The alpha6 nicotinic acetylcholine receptor (nAChR) subunit is involved in nicotine-stimulated dopamine release in the striatum. It is expressed in brain regions and coexpressed with nAChR subtypes implicated in nicotine dependence behaviors; hence, this subunit may play a role in nicotine dependence. Using the alpha6-selective antagonist alpha-conotoxin H9A;L15A (MII[H9A;L15A]), we determined the role of alpha6* nAChRs in the pharmacological and behavioral effects of nicotine. We measured effects of pretreatment with MII[H9A;L15A] on analgesia, locomotion, and body temperature after a single injection of nicotine. Effects of MII[H9A;L15A] on nicotine reward were measured using the conditioned place preference (CPP) paradigm. We further measured physical (somatic signs and hyperalgesia) and affective [anxiety-related behavior and conditioned place aversion (CPA)] nicotine withdrawal behaviors after extended nicotine exposure. Results showed that MII[H9A;L15A] did not block acute nicotine effects on the behaviors measured. Conversely, MII[H9A:l15A] blocked the expression of nicotine CPP, as well as withdrawal-associated CPA and anxiety-related behavior in the elevated plus maze, but not withdrawal-induced somatic signs or hyperalgesia. These results suggest a role for the alpha6 nAChR subunit in nicotine reward and affective nicotine withdrawal but not acute nicotine-induced or physical withdrawal behaviors.

Nicotine-induced upregulation of nicotinic receptors: underlying mechanisms and relevance to nicotine addiction

A major hurdle in defining the molecular biology of nicotine addiction has been characterizing the different nicotinic acetylcholine receptor (nAChR) subtypes in the brain and how nicotine alters their function. Mounting evidence suggests that the addictive effects of nicotine, like other drugs of abuse, occur through interactions with its receptors in the mesolimbic dopamine system, particularly ventral tegmental area (VTA) neurons, where nicotinic receptors act to modulate the release of dopamine. The molecular identity of the nicotinic receptors responsible for drug seeking behavior, their cellular and subcellular location and the mechanisms by which these receptors initiate and maintain addiction are poorly defined. In this commentary, we review how nicotinic acetylcholine receptors (nAChRs) are upregulated by nicotine exposure, the potential posttranslational events that appear to cause it and how upregulation is linked to nicotine addiction.

beta2-Nicotinic acetylcholine receptor availability during acute and prolonged abstinence from tobacco smoking

CONTEXT

Available levels of nicotinic acetylcholine receptors containing the beta(2) subunit (beta(2)*-nAChR) are higher in recently abstinent tobacco smokers compared with participants who never smoked. Variations in beta(2)*-nAChR availability during the course of abstinence may be related to the urge to smoke, the extent of nicotine withdrawal, and successful abstinence.

OBJECTIVE

To examine changes in beta(2)*-nAChR availability during acute and prolonged abstinence from tobacco smoking and to determine how changes in beta(2)*-nAChR availability were related to clinical features of tobacco smoking.

DESIGN

Tobacco smokers participated in up to 4 iodide 123-labeled 5-iodo-A-85380 ([(123)I]5-IA) single-photon emission computed tomography (SPECT) scans during abstinence at 1 day (n = 7) and 1 (n = 17), 2 (n = 7), 4 (n = 11), and 6 to 12 (n = 6) weeks. Age-matched nonsmokers participated in a single [(123)I]5-IA SPECT scan. All participants completed 1 magnetic resonance imaging study.

SETTING

Academic imaging center.

PARTICIPANTS

Tobacco smokers (n = 19) and an age-matched nonsmoker comparison group (n = 20). Main Outcome Measure The [(123)I]5-IA SPECT images were converted to distribution volume and were analyzed using regions of interest.

RESULTS

Compared with nonsmokers, beta(2)*-nAChR availability in the striatum, cortex, and cerebellum of smokers was not different at 1 day of abstinence, was significantly higher at 1 week of abstinence, and was not different at 4 or at 6 to 12 weeks of abstinence. In smokers, beta(2)*-nAChR availability was significantly lower in the cortex and cerebellum at 6 to 12 weeks compared with 1 week of abstinence. In addition, cerebellar beta(2)*-nAChR availability at 4 weeks of abstinence was positively correlated with craving on the day of the SPECT scan.

CONCLUSIONS

These data suggest that higher beta(2)*-nAChR availability persists up to 1 month of abstinence and normalizes to nonsmoker levels by 6 to 12 weeks of abstinence from tobacco smoking. These marked and persistent changes in beta(2)*-nAChR availability may contribute to difficulties with tobacco cessation.

Rats self-administer intravenous nicotine delivered in a novel smoking-relevant procedure: effects of dopamine antagonists

Attempts to explain tobacco addiction have relied heavily on the assumption that each cigarette puff delivers a bolus of nicotine to the brain within seconds. However, nicotine transits from lungs to brain much more gradually than once thought. Nevertheless, animal self-administration studies continue to use rapid (e.g., <3-s) infusions, as well as high unit doses of nicotine (e.g., 15-30 microg/kg/infusion), each equivalent to one to two cigarettes. Here, we report that nicotine is self-administered across a range of infusion durations (3, 30, 60, and 120 s) in rats. Slow (30-s) infusions were preferred over fast (nominal 3-s) infusions and were self-administered across several reinforcement schedules, at doses as low as 3 microg/kg/infusion, equivalent to one to two puffs. A conventional "fast/high" self-administration procedure (3 s-30 microg/kg/infusion) was then compared with our new "slow/low" procedure (30 s-3 microg/kg/infusion) in rats trained on a progressive ratio schedule and acutely challenged with dopamine receptor antagonists. The D(1) antagonist R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine (SCH 23390) (6-25 microg/kg s.c.) reduced intake in both procedures and in rats self-administering cocaine (0.5 mg/kg/infusion). The D(2) antagonists spiperone (3-30 microg/kg s.c.) and sulpiride (5-20 mg/kg i.p.) increased intake of fast/high nicotine and cocaine, but markedly reduced intake of slow/low nicotine. In a final test, in which only infusion speed was varied, an acute spiperone challenge produced the same differential effect on nicotine self-administration. In conclusion, our new slow/low nicotine self-administration procedure, designed to better mimic smoking-associated nicotine intake, is pharmacologically distinct from the conventional fast delivery/high-dose procedure.

Effects of mecamylamine on nicotine-induced conditioned hyperactivity and sensitization in differentially reared rats

Rats reared in an enriched condition (EC) display less sensitization to nicotine than rats reared in an impoverished condition (IC). However, it is unknown what effect differential rearing has on nicotine-induced conditioned hyperactivity. The present study determined whether differential rearing affects nicotine-induced conditioned hyperactivity. This study also examined the effects of mecamylamine on conditioned hyperactivity and sensitization. EC, IC, and social condition (SC) rats were reared from 21 to 51 days of age before receiving repeated nicotine injections (.4 mg/kg) prior to 1-h locomotor sessions. Following the conditioned-hyperactivity test, rats received additional training sessions followed by a drug-free rest period before the sensitization test. Mecamylamine (1.0 mg/kg) was administered prior to the conditioned-hyperactivity test and sensitization test. Nicotine treatment resulted in sensitization and conditioned hyperactivity in all differential rearing groups. EC rats displayed less locomotor activity in response to nicotine than both IC and SC rats. Pretreatment with mecamylamine blocked the expression of conditioned hyperactivity only in EC and SC rats and attenuated sensitization in all three rearing groups. These findings suggest that environmental enrichment may alter nicotinic acetylcholine receptors during development and may be a protective factor in the initiation and relapse of smoking behavior.

Effects of subunit selective nACh receptors on operant ethanol self-administration and relapse-like ethanol-drinking behavior

RATIONALE AND OBJECTIVES

The sensitivity to ethanol central effects is partially determined by the subunit composition of brain nicotinic acetylcholine receptors (nAChRs). Thus, the effects of intraventral tegmental area (VTA) administration of the nicotinic subunit-specific antagonist, alpha-conotoxin MII (alphaCtxMII, alpha(3)beta(2)*, beta(3)*, alpha(6)*), were compared to those of systemic mecamylamine (MEC, an allosteric negative modulator of the nAChR), dihydro-beta-erythroidine (DHbetaE, alpha(4)beta(2)*), and methyllycaconitine (MLA, alpha(7)*) to elucidate involvement of different subunits of nAChRs in operant ethanol self-administration and relapse-like activation of ethanol consumption after ethanol deprivation in rats.

METHODS

The effects of drugs were studied in rats trained for operant oral self-administration of ethanol (FR = 1). For ethanol deprivation, trained animals were subjected to a period of alcohol deprivation for 10 days. alphaCtxMII was given directly into the VTA through implanted permanent intracranial cannulae, whereas MEC, DHbetaE, and MLA were administered systemically.

RESULTS

alphaCtxMII reduced operant ethanol self-administration and blocked the deprivation-induced relapse-like ethanol consumption. MEC reduced operant ethanol self-administration and inhibited the deprivation-induced increase in alcohol consumption. DHbetaE did not alter ethanol self-administration in the lower-dose range but inhibited ethanol intake at a higher dose (4 mg/kg), although this effect might have been nonspecific. MLA failed to block self-administration of ethanol and relapse-like drinking after deprivation.

CONCLUSIONS

Our results indicate that nAChRs are involved in the modulation of operant alcohol self-administration and relapse-like alcohol drinking behavior in rats. Our observations support the working hypothesis that systemically active selective ligands for nAChR alpha(3)beta(2)*, beta(3), and/or alpha(6)* receptor subunits might be of therapeutic value for the treatment of alcoholism.

Molecular and cellular mechanisms of action of nicotine in the CNS

Nicotine achieves its psychopharmacological effects by interacting with nicotinic acetylcholine receptors (nAChRs) in the brain. There are numerous subtypes of nAChR that differ in their properties, including their sensitivity to nicotine, permeability to calcium and propensity to desensitise. The nAChRs are differentially localised to different brain regions and are found on presynaptic terminals as well as in somatodendritic regions of neurones. Through their permeability to cations, these ion channel proteins can influence both neuronal excitability and cell signalling mechanisms, and these various responses can contribute to the development or maintenance of dependence. However, many questions and uncertainties remain in our understanding of these events and their relevance to tobacco addiction. In this chapter, we briefly overview the fundamental characteristics of nAChRs that are germane to nicotine's effects and then consider the cellular responses to acute and chronic nicotine, with particular emphasis on dopamine systems because they have been the most widely studied in the context of nicotine dependence. Where appropriate, methodological aspects are critically reviewed.

The neuronal pathways mediating the behavioral and addictive properties of nicotine

This chapter considers the neurobiological mechanisms that are thought to mediate the reinforcing or rewarding properties of nicotine. It focuses on the data (derived principally from studies with experimental animals) showing that nicotine, like other drugs of dependence, stimulates the mesolimbic dopamine (DA) neurones that project to the nucleus accumbens and that these effects play a pivotal role in the biology underlying nicotine dependence. The reinforcing or rewarding properties of nicotine are thought to be associated particularly with the increase in DA overflow evoked in the shell subdivision of the accumbens. However, behavioural studies suggest that these properties of nicotine in experimental animals do not seem to be sufficiently potent to explain the powerful addiction to tobacco experienced by most habitual smokers. This chapter also considers the biological mechanisms that mediate the effects of cues and stimuli associated with the presentation of nicotine, which are thought to contribute significantly to the powerful addictive properties of tobacco smoke.

Genetic variation in the dopamine pathway and smoking cessation

Twin and family studies have established that genetic factors account for much of the variation in tobacco dependence. Therefore, identification of genetic variants predictive of successful smoking cessation has implications for the future prospect of personalized smoking cessation therapies. Converging data implicate the dopamine pathway as an important neural substrate for tobacco dependence. Several candidate genes within the dopamine pathway (e.g., DRD2 and COMT) have been reported to be associated with the efficacy of bupropion and nicotine replacement therapy, and others (e.g., SLC6A3 and DRD4) have been reported to be associated with smoking cessation independent of pharmacotherapy. However, few of these candidate genes are present within regions of suggestive or significant linkage or overlap with genome-wide linkage or association studies of tobacco dependence or smoking cessation. Future studies should seek to replicate genome-wide association analyses with individual-level genotyping, and use better-defined smoking cessation phenotypes. Once robust evidence for association is established, which may take several more years, further research into the likely cost-effectiveness, feasibility and acceptability of personalized medicine for smoking cessation will be necessary before it can be translated into practice.

The novel nicotinic receptor antagonist N,N'-dodecane-1,12-diyl-bis-3-picolinium dibromide decreases nicotine-induced dopamine metabolism in rat nucleus accumbens

The current study examined the effect of the novel nicotinic acetylcholine receptor antagonist, N,N'-dodecane-1,12-diyl-bis-3-picolinium dibromide (bPiDDB), on nicotine-induced dopamine metabolism in rat nucleus accumbens, striatum and medial prefrontal cortex. Acute nicotine (0.5 mg/kg, s.c.) produced an increase in the content of dihydroxyphenylacetic acid (DOPAC) in nucleus accumbens, but not in striatum or medial prefrontal cortex. Pretreatment with bPiDDB (1 or 3 mg/kg, s.c.) dose-dependently inhibited the nicotine-induced increase in DOPAC content in nucleus accumbens. These results indicate that bPiDDB inhibits the nicotine-induced increase in DOPAC in reward-relevant brain region targeting nicotinic acetylcholine receptors.

Chronic nicotine modifies the effects of morphine on extracellular striatal dopamine and ventral tegmental GABA

Previously, we have shown that 7-week oral nicotine treatment enhances morphine-induced behaviors and dopaminergic activity in the mouse brain. In this study, we further characterized the nicotine-morphine interaction in the mesolimbic and nigrostriatal dopaminergic systems, as well as in the GABAergic control of these systems. In nicotine-pretreated mice, morphine-induced dopamine release in the caudate putamen and nucleus accumbens was significantly augmented, as measured by microdialysis. Chronic nicotine treatment did not change basal extracellular concentrations of dopamine and its metabolites in the caudate putamen and nucleus accumbens, nor did it affect the rate of dopamine synthesis, as assessed by 3-hydroxybenzylhydrazine dihydrochloride-induced DOPA accumulation. GABAergic control of dopaminergic activity was studied by measuring extracellular GABA in the presence of nipecotic acid, an inhibitor of GABA uptake. Acute (0.3 mg/kg or 0.5 mg/kg i.p.) and chronic nicotine, as well as morphine (15 mg/kg s.c.) in control mice decreased nipecotic acid-induced increase in extracellular GABA in the ventral tegmental area/substantia nigra (VTA/SN). In contrast, in nicotine-treated mice, morphine increased GABA levels in the presence of nipecotic acid. We did not find any alterations in GABA(B)-receptor function after chronic nicotine treatment. Thus, our data show that chronic nicotine treatment sensitizes dopaminergic systems to morphine and affects GABAergic systems in the VTA/SN.

Subtypes of nicotinic acetylcholine receptors in nicotine reward, dependence, and withdrawal: evidence from genetically modified mice

Neuronal nicotinic acetylcholine receptors (nAChRs) can regulate the activity of many neurotransmitter pathways throughout the central nervous system and are considered to be important modulators of cognition and emotion. nAChRs are also the primary site of action in the brain for nicotine, the major addictive component of tobacco smoke. nAChRs consist of five membrane-spanning subunits (alpha and beta isoforms) that can associate in various combinations to form functional nAChR ion channels. Owing to a dearth of nAChR subtype-selective ligands, the precise subunit composition of the nAChRs that regulate the rewarding effects of nicotine and the development of nicotine dependence are unknown. The advent of mice with genetic nAChR subunit modifications, however, has provided a useful experimental approach to assess the contribution of individual subunits in vivo. Here, we review data generated from nAChR subunit knockout and genetically modified mice supporting a role for discrete nAChR subunits in nicotine reinforcement and dependence processes. Importantly, the rates of tobacco dependence are far higher in patients suffering from comorbid psychiatric illnesses compared with the general population, which may at least partly reflect disease-associated alterations in nAChR signaling. An understanding of the role of nAChRs in psychiatric disorders associated with high rates of tobacco addiction, therefore, may reveal novel insights into mechanisms of nicotine dependence. Thus, we also briefly review data generated from genetically modified mice to support a role for discrete nAChR subunits in anxiety disorders, depression, and schizophrenia.

Chronic nicotine exposure has dissociable behavioural effects on control and beta2-/- mice

Nicotine exerts beneficial effects on various neurological and psychiatric pathologies, yet its effects on cognitive performance remain unclear. Mice lacking the beta2 subunit of the nicotinic receptor (beta2-/-) show characteristic deficits in executive functions and are suggested as reliable animal models for some specific endophenotypes of human pathologies, notably ADHD. We use beta2-/- and their controls to investigate the consequences of chronic nicotine exposure on cognitive behaviour. We show that in control mice, this treatment elicits somewhat slight effects, particularly affecting nocturnal activity and self-grooming. By contrast, in beta2-/- mice, chronic nicotine treatment had restorative effects on exploratory behaviour in the open-field and affected rearing, but did not modify motor functions. We confirmed that beta2-/- mice exhibit impaired exploratory and social behaviour, and further demonstrated their nocturnal hyperactivity. These data support the proposal that beta2-/- mice represent a relevant model for cognitive disorders in humans and that nicotine administered chronically at low dose may relieve some of these.

Opposing actions of chronic stress and chronic nicotine on striatal function in mice

Stress is a major risk factor in drug addiction development and relapse. Virtually all drugs of abuse act by increasing extracellular dopamine levels in the striatum. To gain an understanding of the interaction between stress and drug exposure, we studied the effects of concomitant chronic nicotine and chronic stress exposure on mouse striatal dopamine levels. C57Bl6/J mice were treated with nicotine in the drinking water or control solution for at least 6 weeks. Some mice were chronically stressed by daily exposure to cold, shaking or restrain. Nicotine-treated mice showed up-regulation of epibatidine binding in several brain regions. In mice treated with both chronic nicotine and stress, epibatidine binding was increased in all studied areas except the dorsal striatum. Therefore, microdialysis was used to measure extracellular dopamine levels in the dorsal striatum of mice chronically treated with nicotine, stress, or both. To have a measure of striatal response to different challenges, we performed microdialysis after acute injection of saline, nicotine, and cocaine. Chronic nicotine enhanced nicotine-dependent dopamine release, while chronic stress blunted the response to cocaine. When mice were subjected to both chronic nicotine and chronic stress, nicotine- and cocaine-dependent dopamine release was undistinguishable from that of control animals. In conclusion, our data suggest that chronic stress and chronic nicotine counteract each other's effect on dopamine release in the striatum. This effect might be mediated by changes in nicotinic acetylcholine receptor up-regulation. This "normalization" of striatal function when both nicotine and stress are present might help explain the comorbidity between stress-related disorders and drug abuse.

Cigarette smoke and dopaminergic system

It has been reported that there is an ameliorative effect of cigarette smoking on certain neurological responses and neurodegenerative disorders. The purpose of this study was to examine the neurochemical and neurobehavioral response of cigarette smoke (CS) in the adult male guinea pig brain. Both acute and chronic CS exposure enhanced locomotor behavior and caused a decrease in midbrain dopamine (DA) levels and corresponding increase in 3,4-dihydroxyphenylacetic acid (DOPAC) levels. In addition, CS caused a significant increase in the protein levels of the dopamine D1 and D2 receptors. CS caused a significant increase in the binding capacity of the D1 receptor and a significant decrease in the binding capacity of D2. Furthermore, CS caused a significant increase in the binding capacity of the dopamine transporter (DAT). The mechanism by which cigarette smoke exposure increases locomotor activity remains to be elucidated but may include modulation of dopamine neuron activity that emerges after repeated direct smoke exposure.

N,N'-Alkane-diyl-bis-3-picoliniums as nicotinic receptor antagonists: inhibition of nicotine-evoked dopamine release and hyperactivity

The current study evaluated a new series of N,N'-alkane-diyl-bis-3-picolinium (bAPi) analogs with C6-C12 methylene linkers as nicotinic acetylcholine receptor (nAChR) antagonists, for nicotine-evoked [3H]dopamine (DA) overflow, for blood-brain barrier choline transporter affinity, and for attenuation of discriminative stimulus and locomotor stimulant effects of nicotine. bAPi analogs exhibited little affinity for alpha4beta2* (* indicates putative nAChR subtype assignment) and alpha7* high-affinity ligand binding sites and exhibited no inhibition of DA transporter function. With the exception of C6, all analogs inhibited nicotine-evoked [3H]DA overflow (IC50 = 2 nM-6 microM; Imax = 54-64%), with N,N'-dodecane-1,12-diyl-bis-3-picolinium dibromide (bPiDDB; C12) being most potent. bPiDDB did not inhibit electrically evoked [3H]DA overflow, suggesting specific nAChR inhibitory effects and a lack of toxicity to DA neurons. Schild analysis suggested that bPiDDB interacts in an orthosteric manner at nAChRs mediating nicotine-evoked [3H]DA overflow. To determine whether bPiDDB interacts with alpha-conotoxin MII-sensitive alpha6beta2-containing nAChRs, slices were exposed concomitantly to maximally effective concentrations of bPiDDB (10 nM) and alpha-conotoxin MII (1 nM). Inhibition of nicotine-evoked [3H]DA overflow was not different with the combination compared with either antagonist alone, suggesting that bPiDDB interacts with alpha6beta2-containing nAChRs. C7, C8, C10, and C12 analogs exhibited high affinity for the blood-brain barrier choline transporter in vivo, suggesting brain bioavailability. Although none of the analogs altered the discriminative stimulus effect of nicotine, C8, C9, C10, and C12 analogs decreased nicotine-induced hyperactivity in nicotine-sensitized rats, without reducing spontaneous activity. Further development of nAChR antagonists that inhibit nicotine-evoked DA release and penetrate brain to antagonize DA-mediated locomotor stimulant effects of nicotine as novel treatments for nicotine addiction is warranted.

Enhanced dopamine release by nicotine in cigarette smokers: a double-blind, randomized, placebo-controlled pilot study

Previous studies of smoking on dopamine release in humans were investigated only in smokers. Using nicotine gum, we examined the effect of nicotine on dopamine release in smokers and non-smokers and its relation to the degree of nicotine dependence. Smokers and non-smokers participated in a double-blind, randomized, placebo-controlled cross-over study. They participated in two PET measurements with [11C]raclopride, in which they received either nicotine or placebo. Changes in [11C]raclopride non-displaceable binding potential (BPND) following nicotine administration were quantified. Smokers showed significant decrease in BP in the striatum following nicotine administration, but non-smokers did not show such a decrease. The BPND difference between the two scanning sessions was correlated with the degree of nicotine dependence. The BPND difference might reflect enhanced dopamine release in smokers and the reinforced effect of nicotine. These data suggest the feasibility of our gum method as well as the importance of the degree of dependence in future studies of the nicotine effect on the dopamine system.

Methylphenidate enhances the abuse-related behavioral effects of nicotine in rats: intravenous self-administration, drug discrimination, and locomotor cross-sensitization

Stimulant drugs, including D-amphetamine, cocaine, and methylphenidate, increase cigarette smoking in controlled human laboratory experiments. Although the mechanism(s) underlying this effect are unknown, it is possible that stimulants may enhance directly the abuse-related effects of nicotine. In the present study, we characterized the behavioral pharmacological interactions between methylphenidate and nicotine in the intravenous self-administration, drug discrimination, and locomotor cross-sensitization procedures. Adult male Sprague-Dawley rats were trained to respond for intravenous nicotine (0.01 or 0.03 mg/kg/infusion) or sucrose, and the acute effects of methylphenidate (1.25-10 mg/kg) were determined; in addition, separate groups of rats were treated with methylphenidate (2.5 mg/kg) or saline before 12 consecutive nicotine (0.03 mg/kg/infusion) self-administration sessions. Next, the discriminative stimulus effects of nicotine (0.03-0.3 mg/kg) and methylphenidate (1.25-10 mg/kg), alone and in combination with a low nicotine dose (0.056 mg/kg), were tested in nicotine-trained rats. Finally, the locomotor effect of repeated methylphenidate (2.5 mg/kg) was tested in rats previously treated with nicotine (0.2-0.8 mg/kg). Results indicated that acute methylphenidate increased the rate of nicotine self-administration at doses that reduced sucrose-maintained responding; furthermore, tolerance to this effect was not apparent following repeated methylphenidate. Methylphenidate, while not substituting for nicotine alone, dose-dependently enhanced the discriminative stimulus effect of a low nicotine dose. In addition, repeated nicotine exposure promoted the development of locomotor sensitization to methylphenidate. Taken together with recent clinical findings, these results suggest that methylphenidate may enhance the abuse-related behavioral effects of nicotine, perhaps increasing vulnerability to tobacco dependence.

Effects of acute tyrosine/phenylalanine depletion on the selective processing of smoking-related cues and the relative value of cigarettes in smokers

RATIONALE

Acute tyrosine/phenylalanine depletion (ATPD) is a validated neurobiological challenge that results in reduced dopaminergic neurotransmission, allowing examination of the effects of a hypodopaminergic state on craving-related processes.

OBJECTIVES

We studied 16 nonabstaining smokers (>10 cigarettes/day; 9 males; age 20-33 years) to whom was administered a tyrosine/phenylalanine-free mixture (TYR/PHE-free) and a balanced amino acid mixture (BAL) in a double-blind, counterbalanced, crossover design.

METHODS

Subjective cigarette craving, attentional bias to smoking-related word cues, relative value of cigarettes, negative mood, and expired carbon monoxide (CO) levels were measured at various timepoints through 300 min. Participants smoked at hourly intervals to prevent acute nicotine withdrawal during testing.

RESULTS

The TYR/PHE-free mixture, as compared to the BAL mixture, was associated with a greater increase in CO levels from baseline (p = 0.01). Adjusting for the potential confounding influence of between-condition differences in CO levels across time, TYR/PHE-free mixture was associated with increased demand for cigarettes (p = 0.01) and decreased attentional bias toward smoking-related words (p = 0.003). There were no significant differences between conditions in either subjective craving or depressed or anxious mood (p values > 0.05).

CONCLUSION

Among nonabstaining daily smokers, acute dopaminergic depletion via ATPD may influence smoking behavior and indices of smoking-related motivation, such as attentional bias to smoking cues and relative cigarette value, which are not readily captured by subjective craving.

Molecular mechanisms underlying the motivational effects of nicotine

In addition to the primary rewarding properties of nicotine and the alleviation of withdrawal symptoms, cues associated with smoking are critical contributors to maintenance of smoking behavior. Nicotine-paired cues are also critical for precipitating relapse after smoking cessation. An accumulation of evidence suggests that repeated exposure to tobacco, including the primary psychoactive ingredient, nicotine, changes brain neurochemistry in a way that promotes the control that cues associated with smoking or other rewards have over behavior. This chapter will consider the neurochemical mechanisms underlying these neuroadaptations. Targeting these molecular alterations may provide novel treatments for smoking cessation.

It is not "either/or": activation and desensitization of nicotinic acetylcholine receptors both contribute to behaviors related to nicotine addiction and mood

Nicotine can both activate and desensitize/inactivate nicotinic acetylcholine receptors (nAChRs). An ongoing controversy in the field is to what extent the behavioral effects of nicotine result from activation of nAChRs, and to what extent receptor desensitization is involved in these behavioral processes. Recent electrophysiological studies have shown that both nAChR activation and desensitization contribute to the effects of nicotine in the brain, and these experiments have provided cellular mechanisms that could underlie the contribution of both these processes to nicotine-mediated behaviors. For instance, desensitization of nAChRs may contribute to the salience of environmental cues associated with smoking behavior and activation and desensitization of nAChRs may contribute to both primary and conditioned drug reward. Similarly, studies of the antidepressant-like effects of nicotinic agents have revealed a balance between activation and desensitization of nAChRs. This review will examine the evidence for the contribution of these two very different consequences of nicotine administration to behaviors related to nicotine addiction, including processes related to drug reinforcement and affective modulation. We conclude that there are effects of nAChR activation and desensitization on drug reinforcement and affective behavior, and that both processes are important in the behavioral consequences of nicotine in tobacco smoking.

Region-specific effects of N,N'-dodecane-1,12-diyl-bis-3-picolinium dibromide on nicotine-induced increase in extracellular dopamine in vivo

BACKGROUND AND PURPOSE

Systemic administration of N,N'-dodecane-1,12-diyl-bis-3-picolinium dibromide (bPiDDB), an antagonist of nicotinic acetylcholine receptors (nAChRs) attenuated the nicotine-induced increase in dopamine levels in nucleus accumbens (NAcc).

EXPERIMENTAL APPROACH

Using in vivo microdialysis, we investigated the effects of local perfusion of the novel nAChR antagonist bPiDDB into the NAcc or ventral tegmental area (VTA) on increased extracellular dopamine in NAcc, induced by systemic nicotine. We also examined the concentration-dependent effects of bPiDDB on the acetylcholine (ACh)-evoked response of specific recombinant neuronal nAChR subtypes expressed in Xenopus oocytes, using electrophysiological methods.

KEY RESULTS

Nicotine (0.4 mg kg(-1), s.c.) increased extracellular dopamine in NAcc, which was attenuated by intra-VTA perfusion of mecamylamine (100 microM). Intra-VTA perfusion of bPiDDB (1 and 10 microM) reduced nicotine-induced increases in extracellular dopamine in NAcc. In contrast, intra-NAcc perfusion of bPiDDB (1 or 10 microM) failed to alter the nicotine-induced increase in dopamine in NAcc. Intra-VTA perfusion of bPiDDB alone did not alter basal dopamine levels, compared to control, nor the increased dopamine in NAcc following amphetamine (0.5 mg kg(-1), s.c.). Using Xenopus oocytes, bPiDDB (0.01-100 microM) inhibited the response to ACh on specific combinations of rat neuronal nAChR subunits, with highest potency at alpha3beta4beta3 and lowest potency at alpha6/3beta2beta3.

CONCLUSIONS AND IMPLICATIONS

bPiDDB-Sensitive nAChRs involved in regulating nicotine-induced dopamine release are located in the VTA, rather than in the NAcc. As bPiDDB has properties different from the prototypical nAChR antagonist mecamylamine, further development may lead to novel nAChR antagonists for the treatment of tobacco dependence.

Exposure to nicotine and sensitization of nicotine-induced behaviors

Evidence for an important link between sensitization of midbrain dopamine (DA) neuron reactivity and enhanced self-administration of amphetamine and cocaine has been reported. To the extent that exposure to nicotine also sensitizes nucleus accumbens DA reactivity, it is likely that it will also impact subsequent drug taking. It is thus necessary to gain an understanding of the long-term effects of exposure to nicotine on nicotinic acetylcholine receptors (nAChRs), neuronal excitability and behavior. A review of the literature is presented in which different regimens of nicotine exposure are assessed for their effects on upregulation of nAChRs, induction of LTP in interconnected midbrain nuclei and development of long-lasting locomotor and DA sensitization. Exposure to nicotine upregulates nAChRs and nAChR currents and produces LTP of excitatory inputs to midbrain DA neurons. These effects appear in the hours to days following exposure. Exposure to nicotine also leads to long-lasting sensitization of nicotine's nucleus accumbens DA and locomotor activating effects. These effects appear days to weeks after drug exposure. A model is proposed in which nicotine exposure regimens that produce transient nAChR upregulation and LTP consequently produce long-lasting sensitization of midbrain DA neuron reactivity and nicotine-induced behaviors. These neuroadaptations are proposed to constitute critical components of the mechanisms underlying the initiation, maintenance and escalation of drug use.

Effects of acute tyrosine depletion on subjective craving and selective processing of smoking-related cues in abstinent cigarette smokers

We investigated the impact of the administration of a tyrosine-depleting amino acid mixture compared to a balanced mixture on measures of mood, craving and selective processing of smoking-related cues in healthy cigarette smokers instructed to abstain from smoking for 12 h prior to, and during, the experiment. A modified stroop task was used to index selective processing of smoking-related cues. We observed evidence for an increase in subjective craving among males, and an attenuation of the selective processing of smoking-related cues compared to control cues among females, in the tyrosine-depleting condition compared to the balanced condition. No effects of mixture were observed on measures of subjective mood. These results tentatively support for the role of dopaminergic neurotransmission in mediating the response of cigarette smokers to smoking-related cues. In addition, these results also provide further evidence for sex differences in the factors that maintain cigarette smoking, in particular with respect to conditioned reinforcement of smoking behaviour, and suggest that the relationship between subjective craving and selective processing of smoking-related cues may differ in males and females.

Comparison of the effects of bupropion and nicotine on locomotor activation and dopamine release in vivo

Bupropion is an atypical anti-depressant that is approved for smoking cessation. In addition to inhibiting dopamine reuptake, bupropion has been reported to block nicotinic acetylcholine receptors in vitro, and this action might contribute to its efficacy for smoking cessation. In this study we investigated if nicotinic receptor-mediated responses in vivo are decreased in the presence of a behaviorally effective dose of bupropion. In separate experiments we measured locomotor activation and dopamine overflow in the nucleus accumbens core, using in vivo microdialysis in freely moving rats. Bupropion (30 mg/kg i.p.) increased locomotor activity, which remained elevated for up to 2 h. Nicotine (0.4 mg/kg s.c.) also increased locomotor activity but for a shorter duration. When given 20 min after bupropion, hyperlocomotion was significantly enhanced, compared to the response to either nicotine or bupropion alone, consistent with the effects of the two drugs being additive. Systemic administration of bupropion (30 mg/kg i.p.) also elicited a significant increase in dopamine overflow (113+/-16% above basal levels). Nicotine (3 mM; delivered into the nucleus accumbens core via the microdialysis probe) increased dopamine overflow by 126 +/- 35%. Nicotine delivered during the response to bupropion resulted in enhanced dopamine overflow of 294 +/- 50%, also consistent with the actions of the two drugs being additive. This study suggests that behaviorally effective concentrations of bupropion in the rat do not diminish the effects of nicotine by blocking nicotinic receptors.

Chronic bupropion attenuated the anhedonic component of nicotine withdrawal in rats via inhibition of dopamine reuptake in the nucleus accumbens shell

Bupropion, a dopamine reuptake inhibitor, is an effective therapy for smoking cessation, but the behavioral and neurochemical mechanisms mediating its antismoking properties are relatively unknown. To explore the hypothesis that bupropion ameliorates nicotine withdrawal partly by a dopamine-dependent mechanism, we investigated the effects of chronic bupropion on potassium-stimulated dopamine overflow in the nucleus accumbens shell in nicotine-withdrawing rats. We also assessed the effects of chronic bupropion on behavioral aspects of nicotine withdrawal measured by elevations in brain reward thresholds and somatic signs of withdrawal. Rats were treated with nicotine or saline for 7 days and then coadministration of bupropion or saline was initiated. After 14 days of coadministration of bupropion/saline and nicotine/saline, nicotine/saline administration was terminated, whereas bupropion/saline administration continued. These conditions mimic bupropion administration in human smokers. Cessation of nicotine administration in non-bupropion-treated rats elevated reward thresholds reflecting a reward deficit, increased somatic signs and diminished potassium-evoked dopamine overflow in the nucleus accumbens shell. Chronic bupropion lowered reward thresholds and increased potassium-evoked dopamine release regardless of previous nicotine exposure, possibly by inhibition of dopamine reuptake, and thus attenuated the anhedonic and neurochemical effects of nicotine withdrawal. Chronic bupropion blocked withdrawal-associated increased somatic signs. Finally, acute experimenter-administered nicotine enhanced brain reward function equally in all groups, indicating that bupropion does not alter the reward-facilitating effects of experimenter-administered nicotine. In conclusion, the bupropion-induced increase in extracellular dopamine in the nucleus accumbens shell may ameliorate the anhedonia associated with nicotine withdrawal, which in turn may facilitate smoking cessation.

Pharmacogenetic clinical trial of sustained-release bupropion for smoking cessation

This randomized, double-blinded, placebo-controlled trial examined genetic influences on treatment response to sustained-release bupropion for smoking cessation. Smokers of European ancestry (N = 291), who were randomized to receive bupropion or placebo (12 weeks) plus counseling, were genotyped for the dopamine D2 receptor (DRD2-Taq1A), dopamine transporter (SLC6A3 3' VNTR), and cytochrome P450 2B6 (CYP2B6 1459 CT) polymorphisms. Main outcome measures were cotinine-verified point prevalence of abstinence at end of treatment and at 2-, 6-, and 12-month follow-ups post quit date. Using generalized estimating equations, we found that bupropion, compared with placebo, was associated with significantly greater odds of abstinence at all time points (all p values<.01). We found a significant DRD2 x bupropion interaction (B = 1.49, SE = 0.59, p = .012) [corrected] and a three-way DRD2 x bupropion x craving interaction on 6-month smoking cessation outcomes (B = -0.45, SE = 0.22, p = .038), such that smokers with the A2/A2 genotype demonstrated the greatest craving reduction and the highest abstinence rates with bupropion. Furthermore, there was a significant DRD2 x CYP2B6 interaction (B = 1.43, SE = 0.56, p = .01), such that individuals with the DRD2-Taq1 A2/A2 genotype demonstrated a higher odds of abstinence only if they possessed the CYP2B6 1459 T/T or C/T genotype. Because the sample size of this study was modest for pharmacogenetic investigations, the results should be interpreted with caution. Although these results require replication, the data suggest preliminarily that the DRD2-Taq1A polymorphism may influence treatment response to bupropion for smoking cessation and, further, that exploration of gene x gene and gene x craving interactions in future, larger studies may provide mechanistic insights into the complex pharmacodynamics of bupropion.

The subtypes of nicotinic acetylcholine receptors on dopaminergic terminals of mouse striatum

This review summarizes studies that attempted to determine the subtypes of nicotinic acetylcholine receptors (nAChR) expressed in the dopaminergic nerve terminals in the mouse. A variety of experimental approaches has been necessary to reach current knowledge of these subtypes, including in situ hybridization, agonist and antagonist binding, function measured by neurotransmitter release from synaptosomal preparations, and immunoprecipitation by selective antibodies. Early developments that facilitated this effort include the radioactive labeling of selective binding agents, such as [(125)I]-alpha-bungarotoxin and [(3)H]-nicotine, advances in cloning the subunits, and expression and evaluation of function of combinations of subunits in Xenopus oocytes. The discovery of epibatidine and alpha-conotoxin MII (alpha-CtxMII), and the development of nAChR subunit null mutant mice have been invaluable in determining which nAChR subunits are important for expression and function in mice, as well as allowing validation of the specificity of subunit specific antibodies. These approaches have identified five nAChR subtypes of nAChR that are expressed on dopaminergic nerve terminals. Three of these contain the alpha6 subunit (alpha4alpha6beta2beta3, alpha6beta2beta3, alpha6beta2) and bind alpha-CtxMII with high affinity. One of these three subtypes (alpha4alpha6beta2beta3) also has the highest sensitivity to nicotine of any native nAChR that has been studied, to date. The two subtypes that do not have high affinity for alpha-CtxMII (alpha4beta2, alpha4alpha5beta2) are somewhat more numerous than the alpha6* subtypes, but do bind nicotine with high affinity. Given that our first studies detected readily measured differences in sensitivity to agonists and antagonists among these five nAChR subtypes, it seems likely that subtype selective compounds could be developed that would allow therapeutic manipulation of diverse nAChRs that have been implicated in a number of human conditions.

Association study of the nicotinic acetylcholine receptor alpha4 subunit gene, CHRNA4, in attention-deficit hyperactivity disorder

Attention-deficit hyperactivity disorder (ADHD) is a common childhood-onset psychiatric condition with a strong genetic component. Evidence from pharmacological, clinical and animal studies has suggested that the nicotinic system could be involved in the disorder. Previous studies have implicated the nicotinic acetylcholine receptor alpha4 subunit gene, CHRNA4, in ADHD. Particularly, a polymorphism in the exon 2-intron 2 junction of CHRNA4 has been associated with severe inattention defined by latent class analysis. In the current study, we used the transmission disequilibrium test (TDT) to investigate four polymorphisms encompassing this region of CHRNA4 for association with ADHD in a sample of 264 nuclear families from Toronto. No significant evidence of biased transmission was observed for any of the marker alleles for ADHD defined as a categorical trait (all subtypes included), although one haplotype showed marginal evidence of under-transmission. No association was found with the ADHD predominantly inattentive subtype or with symptom dimension scores of inattention. On the contrary, nominally significant evidence of association of individual markers was obtained for the ADHD combined subtype and with teacher-rated hyperactivity-impulsivity scores, with the same haplotype being under-transmitted. Based on our results and others, CHRNA4 may be involved in ADHD; however, its role in ADHD symptomatology remains to be clarified.