Nicotine addiction: insights from recent animal studies

BDNF and nicotine dependence: associations and potential mechanisms

Smoking is the leading preventable cause of death worldwide and tobacco addiction has become a serious public health problem. Nicotine is the main addictive component of tobacco, and the majority of people that smoke regularly develop nicotine dependence. Nicotine addiction is deemed to be a chronic mental disorder. Although it is well known that nicotine binds to the nicotinic acetylcholine receptors (nAChRs) and activates the mesolimbic dopaminergic system (MDS) to generate the pleasant and rewarding effects, the molecular mechanisms of nicotine addiction are not fully understood. Brain-derived neurotrophic factor (BDNF) is the most prevalent growth factor in the brain, which regulates neuron survival, differentiation, and synaptic plasticity, mainly through binding to the high affinity receptor tyrosine kinase receptor B (TrkB). BDNF gene polymorphisms are associated with nicotine dependence and blood BDNF levels are altered in smokers. In this review, we discussed the effects of nicotine on BDNF expression in the brain and summarized the underlying signaling pathways, which further indicated BDNF as a key regulator in nicotine dependence. Further studies that aim to understand the neurobiological mechanism of BDNF in nicotine addcition would provide a valuable reference for quitting smoking and developing the treatment of other addictive substances.

Neurostimulation techniques in the treatment of nicotine dependence: A review

OBJECTIVE

Nicotine dependence accounts for significant mortality, morbidity, and socio-economic burdens. It remains a significant public health concern since it is among the leading causes of mortality worldwide and of preventable deaths in developed countries. Despite the availability of approved medications to treat nicotine dependence, along with cognitive behavioral therapy, only 6% of the total number of smokers who report wanting to quit each year are successful in doing so for more than a month mostly with poor abstinence rates. Urgent therapeutic alternatives are therefore needed such as neurostimulation techniques. The purpose of this review is to describe studies that have evaluated the safety and efficacy of these techniques for the treatment of nicotine dependence.

METHODS

The electronic literature on repetitive transcranial magnetic stimulation, theta-burst stimulation, deep transcranial magnetic stimulation, transcranial direct current stimulation, magnetic seizure therapy, electroconvulsive therapy, cranial electro-stimulation, and deep brain stimulation in the treatment of nicotine addiction were reviewed.

RESULTS

Most of these studies found that some of these neurostimulation techniques are safe and potentially effective in the reduction of craving to nicotine as well as in the reduction of cigarette consumption.

CONCLUSION AND SCIENTIFIC SIGNIFICANCE

Given the promising results of some of the studies particularly with repetitive transcranial magnetic stimulation, theta-burst stimulation, transcranial direct current stimulation and, possibly, deep transcranial magnetic stimulation, future controlled studies with larger samples, and optimal stimulus parameters should be designed to confirm these findings. (Am J Addict 2016;25:436-451).

Donepezil, an acetylcholinesterase inhibitor, attenuates nicotine self-administration and reinstatement of nicotine seeking in rats

Nicotine craving and cognitive impairments represent core symptoms of nicotine withdrawal and predict relapse in abstinent smokers. Current smoking cessation pharmacotherapies have limited efficacy in preventing relapse and maintaining abstinence during withdrawal. Donepezil is an acetylcholinesterase inhibitor that has been shown previously to improve cognition in healthy non-treatment-seeking smokers. However, there are no studies examining the effects of donepezil on nicotine self-administration and/or the reinstatement of nicotine-seeking behavior in rodents. The present experiments were designed to determine the effects of acute donepezil administration on nicotine taking and the reinstatement of nicotine-seeking behavior, an animal model of relapse in abstinent human smokers. Moreover, the effects of acute donepezil administration on sucrose self-administration and sucrose seeking were also investigated in order to determine whether donepezil's effects generalized to other reinforced behaviors. Acute donepezil administration (1.0 or 3.0 mg/kg, i.p.) attenuated nicotine, but not sucrose self-administration maintained on a fixed-ratio 5 schedule of reinforcement. Donepezil administration also dose-dependently attenuated the reinstatement of both nicotine- and sucrose-seeking behaviors. Commonly reported adverse effects of donepezil treatment in humans are nausea and vomiting. However, at doses required to attenuate nicotine self-administration in rodents, no effects of donepezil on nausea/malaise as measured by pica were observed. Collectively, these results indicate that increased extracellular acetylcholine levels are sufficient to attenuate nicotine taking and seeking in rats and that these effects are not due to adverse malaise symptoms such as nausea.

Repetitive transcranial magnetic stimulation of the dorsolateral prefrontal cortex reduces nicotine cue craving

BACKGROUND

Repetitive transcranial magnetic stimulation (rTMS) can noninvasively stimulate the brain and transiently amplify or block behaviors mediated through a region. We hypothesized that a single high-frequency rTMS session over the left dorsolateral prefrontal cortex (DLPFC) would reduce cue craving for cigarettes compared with a sham TMS session.

METHODS

Sixteen non-treatment-seeking, nicotine-dependent participants were randomized to receive either real high-frequency rTMS (10 Hz, 100% resting motor threshold, 5-sec on, 10-sec off for 15 min; 3000 pulses) or active sham (eSham) TMS over the DLPFC in two visits with 1 week between visits. The participants received cue exposure before and after rTMS and rated their craving after each block of cue presentation.

RESULTS

Stimulation of the left DLFPC with real, but not sham, rTMS reduced craving significantly from baseline (64.1±5.9 vs. 45.7±6.4, t = 2.69, p = .018). When compared with neutral cue craving, the effect of real TMS on cue craving was significantly greater than the effect of sham TMS (12.5±10.4 vs. -9.1±10.4; t = 2.07, p = .049). More decreases in subjective craving induced by TMS correlated positively with higher Fagerström Test for Nicotine Dependence score (r = .58, p = .031) and more cigarettes smoked per day (r = .57, p = .035).

CONCLUSIONS

One session of high-frequency rTMS (10 Hz) of the left DLPFC significantly reduced subjective craving induced by smoking cues in nicotine-dependent participants. Additional studies are needed to explore rTMS as an aid to smoking cessation.

Galantamine, an acetylcholinesterase inhibitor and positive allosteric modulator of nicotinic acetylcholine receptors, attenuates nicotine taking and seeking in rats

Current smoking cessation pharmacotherapies have limited efficacy in preventing relapse and maintaining abstinence during withdrawal. Galantamine is an acetylcholinesterase inhibitor that also acts as a positive allosteric modulator of nicotinic acetylcholine receptors. Galantamine has recently been shown to reverse nicotine withdrawal-induced cognitive impairments in mice, which suggests that galantamine may function to prevent relapse in human smokers. However, there are no studies examining whether galantamine administration modulates nicotine self-administration and/or reinstatement of nicotine seeking in rodents. The present experiments were designed to determine the effects of galantamine administration on nicotine taking and reinstatement of nicotine-seeking behavior, an animal model of relapse. Moreover, the effects of galantamine on sucrose-maintained responding and sucrose seeking were also examined to determine whether galantamine's effects generalized to other reinforced behaviors. An inverted U-shaped dose-response curve was obtained when animals self-administered different unit doses of nicotine with the highest responding for 0.03 mg/kg per infusion of nicotine. Acute galantamine administration (5.0 mg/kg, i.p.) attenuated nicotine self-administration when animals were maintained on either a fixed-ratio 5 (FR5) or progressive ratio (PR) schedule of reinforcement. Galantamine administration also attenuated the reinstatement of nicotine-seeking behavior. No significant effects of galantamine on sucrose self-administration or sucrose reinstatement were noted. Acetylcholinesterase inhibitors have also been shown to produce nausea and vomiting in humans. However, at doses required to attenuate nicotine self-administration, no effects of galantamine on nausea/malaise as measured by pica were noted. These results indicate that increased extracellular acetylcholine levels and/or nicotinic acetylcholine receptor stimulation is sufficient to attenuate nicotine taking and seeking in rats and that these effects are reinforcer selective and not due to adverse malaise symptoms such as nausea.

Environmental enrichment alters nicotine-mediated locomotor sensitization and phosphorylation of DARPP-32 and CREB in rat prefrontal cortex

Exposure within an environmental enrichment paradigm results in neurobiological adaptations and decreases the baseline of locomotor activity. The current study determined activation of DARPP-32 (dopamine- and cAMP-regulated phosphoprotein-32) and CREB (cAMP response element binding protein), and locomotor activity in rats raised in enriched (EC), impoverished (IC), and standard (SC) conditions following repeated administration of nicotine or saline. In the saline-control group, the basal phosphorylation state of DARPP-32 at Threonine-34 site (pDARPP-32 Thr34) in the prefrontal cortex (PFC) was lower in EC compared to IC and SC rats, which was positively correlated with their respective baseline activities. While nicotine (0.35 mg/kg, freebase) produced locomotor sensitization across all housing conditions when the nicotine-mediated locomotor activity was expressed as a percent change from their respective saline control, EC rats displayed greater sensitization to nicotine than IC and SC rats. Consistent with the behavioral findings, repeated nicotine injection increased pDARPP-32 Thr34 in PFC of EC and IC rats and in nucleus accumbens of EC rats; however, the magnitude of change from saline control in nicotine-induced enhancement of pDARPP-32 Thr34 in PFC was strikingly increased in EC rats relative to IC rats. Moreover, EC rats had lower basal phosphorylation levels of CREB at serine 133 in PFC and nucleus accumbens compared to IC and SC rats, whereas the nicotine-induced increase in phosphorylated CREB-Ser133 was more pronounced in PFC of EC rats relative to IC and SC rats. Collectively, these findings suggest innovative insights into advancing our understanding of the molecular mechanisms of enrichment-induced changes in the motivational effects of nicotine, and aiding in the identification of new therapeutic strategies for tobacco smokers.

Effects of Medicinal Herb Tea on the Smoking Cessation and Reducing Smoking Withdrawal Symptoms

Medicinal herbs (21 species) were screened for the antioxidant activity and nicotine degradation activity (NDA) in vitro. Eleven of them with higher antioxidant activity and NDA were selected for preparation of the medicinal herb tea (MHT) and the effects of MHT on smoking cessation and reducing smoking withdrawal symptoms were evaluated in 100 male human smokers.

Among these medicinal herbs, Eugenia aromaticum and Astragalus membranaceus Bunge showed the highest antioxidant activity ( IC 50 of 30.0 μg/mL) and NDA (1.81), respectively. MHT showed relatively high antioxidant activity ( IC 50 of 50.6 μg/mL) and NDA (1.23). The urinary cotinine level, a metabolite of nicotine, increased in the first 2 weeks and greatly decreased from the 2nd to 4th week in the MHT taking group, which indicates that MHT accelerates the conversion of nicotine into cotinine. Human groups taking MHT for 4 weeks underwent reduced smoking withdrawal symptoms compared to the non-MHT taking subjects, and 38% of subjects taking MHT succeeded in smoking cessation, while only 12% of non-MHT taking subjects succeeded in quitting smoking.

Neurophysiology of Nicotine Addiction

Tobacco use is a major health problem, and nicotine is the main addictive component. Nicotine binds to nicotinic acetylcholine receptors (nAChR) to produce its initial effects. The nAChRs subtypes are composed of five subunits that can form in numerous combinations with varied functional and pharmacological characteristics. Diverse psychopharmacological effects contribute to the overall process of nicotine addiction, but two general neural systems are emerging as critical for the initiation and maintenance of tobacco use. Mesocorticolimbic circuitry that includes the dopaminergic pathway originating in the ventral tegmental area and projecting to the nucleus accumbens is recognized as vital for reinforcing behaviors during the initiation of nicotine addiction. In this neural system β2, α4, and α6 are the most important nAChR subunits underlying the rewarding aspects of nicotine and nicotine self-administration. On the other hand, the epithalamic habenular complex and the interpeduncular nucleus, which are connected via the fasciculus retroflexus, are critical contributors regulating nicotine dosing and withdrawal symptoms. In this case, the α5 and β4 nAChR subunits have critical roles in combination with other subunits. In both of these neural systems, particular nAChR subtypes have roles that contribute to the overall nicotine addiction process.

Assessing the impact of nicotine dependence genes on the risk of facial clefts: An example of the use of national registry and biobank data

BACKGROUND

Maternal smoking during pregnancy has been associated with risk of facial clefts in offspring, but causation has not yet been established. It is possible that the effect of maternal smoking on facial clefts is mediated through genes that are involved in nicotine dependence. Gamma-aminobutyric acid B receptor 2 (GABBR2), dopa decarboxylase (DDC), and cholinergic receptor nicotinic alpha 4 (CHRNA4) are three examples of genes that have previously shown strong associations with nicotine dependence.

METHODS

We used a population-based sample of 377 case-parent trios of cleft lip with or without cleft palate (CL/P) and 762 control-parent trios from Norway (1996-2001) to investigate whether variants in GABBR2, DDC and CHRNA4 are associated with maternal first-trimester smoking and with clefting risk. We used HAPLIN (Gjessing et al. 2006), a statistical software tailored for family-based association tests, to perform haplotype-based analyses on 12 SNPs in these genes (rs10985765, rs1435252, rs3780422, rs2779562, and rs3750344 in GABBR2; rs2060762, rs3757472, rs1451371, rs3735273, and rs921451 in DDC; rs4522666 and rs1044393 in CHRNA4).

RESULTS

When analyzed one at a time, there was little evidence of association between any of the 12 SNPs and maternal first-trimester smoking. In haplotype analyses, however, one copy of the maternal G-G-c-G-c haplotype in DDC was linked with smoking prevalence (odds ratio: 1.5; 95% confidence interval: 1.0-2.1). This same haplotype also increased the risk of isolated CL/P in offspring by 1.5-fold with one copy and 2.4-fold with two copies (P_trend = 0.06). No statistically significant associations were detected with GABBR2 and CHRNA4.

CONCLUSIONS

Despite strong associations previously reported between nicotine dependence and variants in GABBR2, DDC and CHRNA4, these genes were poor predictors of maternal first-trimester smoking in our data. The direct association of the DDC haplotype with CL/P suggests that this haplotype may either have direct effects on clefts or it may influence clefting risks through other yet unexplored risk behavior(s).

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.

Nicotinic receptors and stages of nicotine dependence

Smoking is one of the leading causes of preventable death, where nicotine has been identified as the primary addictive constituent of tobacco. Consequently, there have been extensive investigations into the neuroadaptations that occur as nicotine dependence develops, where numerous neurological systems have been implicated. The focus of this review was on nicotinic acetylcholine receptor neuroadaptations that occur during the development of nicotine dependence. This focus was selected because (1) the nicotinic receptors are the primary binding sites for both nicotine and the most efficacious pharmacological smoking cessation treatments and (2) the receptors are located throughout the brain with considerable neuromodulatory ability. However, there was difficulty associated in outlining the role of nicotinic receptors in the development of nicotine dependence because it comprises a series of stages involving different neurological systems rather than a single state. To address this issue, the review adopts a novel approach and considers the role of nicotinic receptor subtypes at separate stages of the nicotine dependence cycle. This information was then used to examine the nicotinic receptor-related therapeutic mechanisms of three main pharmacological smoking cessation treatments.

α7 Nicotinic Receptor Agonists: Potential Therapeutic Drugs for Treatment of Cognitive Impairments in Schizophrenia and Alzheimer's Disease

Accumulating evidence suggests that α7 nicotinic receptors (α7 nAChRs), a subtype of nAChRs, play a role in the pathophysiology of neuropsychiatric diseases, including schizophrenia and Alzheimer's disease (AD). A number of psychopharmacological and genetic studies shown that α7 nAChRs play an important role in the deficits of P50 auditory evoked potential in patients with schizophrenia, and that (α nAChR agonists would be potential therapeutic drugs for cognitive impairments associated with P50 deficits in schizophrenia. Furthermore, some studies have demonstrated that α7 nAChRs might play a key role in the amyloid-β (Aβ)-mediated pathology of AD, and that α7 nAChR agonists would be potential therapeutic drugs for Aβ deposition in the brains of patients with AD. Interestingly, the altered expression of α7 nAChRs in the postmortem brain tissues from patients with schizophrenia and AD has been reported. Based on all these findings, selective α7 nAChR agonists can be considered potential therapeutic drugs for cognitive impairments in both schizophrenia and AD. In this article, we review the recent research into the role of α7 nAChRs in the pathophysiology of these diseases and into the potential use of novel α7 nAChR agonists as therapeutic drugs.

Comparison of the behavioral effects of cigarette smoke and pure nicotine in rats

Animal models of tobacco dependence typically rely on parenteral administration of pure nicotine. Models using cigarette smoke inhalation might more accurately simulate nicotine exposure in smokers. The primary goal of this study was to validate methods for administering cigarette smoke to rats using exposure conditions that were clinically relevant and also produced brain nicotine levels similar to those produced by behaviorally active doses of pure nicotine. A secondary goal was to begin examining the behavioral effects of smoke. Nose-only exposure (NOE) to smoke for 10-45min or whole-body exposure (WBE) to smoke for 1-4h produced serum nicotine concentrations similar to those in smokers (14-55ng/ml), without excessive carbon monoxide exposure. Daily nicotine (0.1mg/kg, s.c.) induced locomotor sensitization whereas 45-min NOE producing brain nicotine levels within the same range did not. Nicotine 0.125mg/kg s.c. reversed withdrawal from a chronic nicotine infusion as measured by elevations in intracranial self-stimulation thresholds whereas 4-h WBE producing similar brain nicotine levels did not. These data demonstrate the feasibility of delivering cigarette smoke to rats at clinically relevant doses, and provide preliminary evidence that the behavioral effects of nicotine delivered in smoke may differ from those of pure nicotine.

Repeated nicotine administration robustly increases bPiDDB inhibitory potency at alpha6beta2-containing nicotinic receptors mediating nicotine-evoked dopamine release

The novel nicotinic receptor (nAChR) antagonist, N,N'-dodecane-1,12-diyl-bis-3-picolinium dibromide (bPiDDB), and its chemically reduced analog, r-bPiDDB, potently inhibit nicotine-evoked dopamine (DA) release from rat striatal slices. Since tobacco smokers self-administer nicotine repeatedly, animal models incorporating repeated nicotine treatment allow for mechanistic evaluation of therapeutic candidates following neuroadaptive changes. The current study determined the ability of bPiDDB, r-bPiDDB and alpha-conotoxin MII (alpha-CtxMII), a peptide antagonist selective for alpha6beta2-containing nAChRs, to inhibit nicotine-evoked [(3)H]DA release from striatal slices from rats repeatedly administered nicotine (0.4mg/kg for 10 days) or saline (control). Concomitant exposure to maximally effective concentrations of r-bPiDDB (1nM) and alpha-CtxMII (1nM) resulted in inhibition of nicotine-evoked [(3)H]DA release no greater than that produced by either antagonist alone, suggesting that r-bPiDDB inhibits alpha6beta2-containing nAChRs. Repeated nicotine treatment increased locomotor activity, demonstrating behavioral sensitization. Concentration-response curves for nicotine-evoked [(3)H]DA release were not different between nicotine-treated and control groups. Maximal inhibition for alpha-CtxMII was greater following repeated nicotine compared to control (I(max)=90% vs. 62%), with no change in potency. bPiDDB was 3-orders of magnitude more potent in inhibiting nicotine-evoked [(3)H]DA release in nicotine-treated rats compared to control rats (IC(50)=5pM vs. 6nM), with no change in maximal inhibition. Neither a shift to the left in the concentration response nor a change in maximal inhibition was observed for r-bPiDDB following repeated nicotine. Thus, repeated nicotine treatment may differentially regulate the stoichiometry, conformation and/or composition of alpha6beta2-containing nAChRs mediating nicotine-evoked striatal DA release. Therefore, bPiDDB and r-bPiDDB appear to target different alpha6beta2-containing nAChR subtypes.

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.

Smoking cessation pharmacotherapy

Cigarette smoking remains an important risk factor for premature cardiovascular disease and its many complications. There are clear benefits from treating tobacco dependence on the rate of clinical outcomes. In addition to behavioral therapies, various pharmacologic strategies have been developed to help achieve this goal. First-line therapies include nicotine replacement, bupropion and varenicline, a partial nicotine antagonist. Second-line treatments include clonidine and nortriptyline. Additional treatment strategies with less proven efficacy include monoamine oxidase inhibitors, selective serotonin reuptake inhibitors, opioid receptor antagonists, bromocriptine, anti-anxiety drugs, nicotinic receptor antagonists (e.g. mecamylamine) and glucose tablets. Various approaches under investigation include inhibitors of the hepatic P450 enzyme (e.g. methoxsalen), cannabinoid-1 receptor antagonists (e.g. rimonabant), and nicotine vaccines.

Repeated high-frequency transcranial magnetic stimulation over the dorsolateral prefrontal cortex reduces cigarette craving and consumption

AIMS

To evaluate the effect of repeated high-frequency transcranial magnetic stimulation (rTMS) of the left dorsolateral prefrontal cortex (DLPFC), combined with either smoking or neutral cues, on cigarette consumption, dependence and craving.

DESIGN

Participants were divided randomly to real and sham stimulation groups. Each group was subdivided randomly into two subgroups presented with either smoking-related or neutral pictures just before the daily TMS intervention. Ten daily rTMS sessions were applied every week-day and then a maintenance phase was conducted in which rTMS sessions were less frequent.

SETTING

Single-site, out-patient, randomized, double-blind, sham-controlled.

PARTICIPANTS

Forty-eight chronic smokers who smoked at least 20 cigarettes per day and were motivated to quit smoking. Healthy males and females were recruited from the general population using advertisements in newspapers and on internet websites.

INTERVENTION

Ten daily rTMS sessions were administered using a standard figure-8 coil over the DLPFC. Stimulation included 20 trains/day at 100% of motor threshold. Each train consisted of 50 pulses at 10 Hz with an inter-train interval of 15 seconds.

MEASUREMENTS

Cigarette consumption was evaluated objectively by measuring cotinine levels in urine samples and subjectively by participants' self-reports. Dependence and craving were evaluated by standard questionnaires.

FINDINGS

Ten daily rTMS sessions over the DLPFC reduced cigarette consumption and nicotine dependence. Furthermore, treatment blocked the craving induced by daily presentation of smoking-related pictures. However, these effects tended to dissipate over time.

CONCLUSIONS

Multiple high-frequency rTMS of the DLPFC can attenuate nicotine craving.

Role of the dopamine transporter in the action of psychostimulants, nicotine, and other drugs of abuse

A number of studies over the last two decades have demonstrated the critical importance of dopamine (DA) in the behavioral pharmacology and addictive properties of abused drugs. The DA transporter (DAT) is a major target for drugs of abuse in the category of psychostimulants, and for methylphenidate (MPH), a drug used for treating attention deficit hyperactivity disorder (ADHD), which can also be a psychostimulant drug of abuse. Other drugs of abuse such as nicotine, ethanol, heroin and morphine interact with the DAT in more indirect ways. Despite the different ways in which drugs of abuse can affect DAT function, one evolving theme in all cases is regulation of the DAT at the level of surface expression. DAT function is dynamically regulated by multiple intracellular and extracellular signaling pathways and several protein-protein interactions. In addition, DAT expression is regulated through the removal (internalization) and recycling of the protein from the cell surface. Furthermore, recent studies have demonstrated that individual differences in response to novel environments and psychostimulants can be predicted based on individual basal functional DAT expression. Although current knowledge of multiple factors regulating DAT activity has greatly expanded, many aspects of this regulation remain to be elucidated; these data will enable efforts to identify drugs that might be used therapeutically for drug dependence therapeutics.

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

RATIONALE

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

OBJECTIVE

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSIONS

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

The role of the hippocampus in mediating emotional responses to nicotine and cannabinoids: a possible neural substrate for functional interactions

The endocannabinoid system is involved in the regulation of behavioural and physiological stress-related responses. Nicotine exerts complex effects on emotional behaviour, and its withdrawal may result in depressive and anxiogenic-like symptoms. Cannabinoid receptor agonists and nicotine induce biphasic effects in diverse tests of unconditioned anxiety, alter adrenocortical activity and affect hippocampus-dependent contextual fear conditioning. Upon exposure to stressful stimuli, central endocannabinoid and cholinergic systems appear to be activated in key limbic areas such as hippocampus and amygdala, which might contribute to adaptive cognitive and emotional strategies to cope with aversive situations. Numerous studies indicate the existence of functional interactions between nicotine and cannabinoids, particularly in relation to anxiety-related processes. An overlapping distribution of CB1 and nicotinic acetylcholine receptors in the hippocampus is observed and the endocannabinoid system exerts a modulatory role over the hippocampal cholinergic system. In this review, we point to the hippocampus as a relevant neural substrate for cannabinoid-nicotine interactions, notably as regards emotional responses. After a general description of the cannabinoid and nicotinic systems, we review their implications in unconditioned anxiety, depressive-like behaviour and fear conditioning. Then we discuss the role of both systems in modulating stress-induced changes at cellular, endocrine and behavioural levels and their possible involvement in hippocampal neurogenesis. Although we mainly focus on animal data, some relevant human studies are also discussed.

Motivational effects of nicotine as measured in a runway model of drug self-administration

Traditional operant self-administration and conditioned place preference methods have yielded inconsistent results in studies of nicotine reinforcement thereby hindering efforts to identify the neurobiological systems underlying the drug's motivation and reinforcement. This study was designed to assess the motivation of subjects to seek nicotine using a runway self-administration procedure. Male Sprague-Dawley rats (N=67) were trained to run a straight alley for a single daily intravenous injection of nicotine (0.01-0.09 mg/kg/injection) on each of 21 consecutive trials. Run Speed (1/Run Time) served as the dependent measure for the animals' motivation to traverse the alley and enter a goal-box associated with intravenous nicotine administration. Nicotine induced an inverted U-shaped dose-response curve with the 0.03 mg/kg dose producing optimal runway performance over trials. Subjects running for doses larger or smaller than the optimal dose exhibited slower running and took longer to enter the goal-box. Thus, the runway procedure proved to be an effective methodology for reliably assessing the motivation of trained but nondrugged animals to seek intravenous nicotine.

Stress, dysregulation of drug reward pathways, and the transition to drug dependence

This review provides a neuroadaptive perspective regarding the role of the hormonal and brain stress systems in drug addiction with a focus on the changes that occur during the transition from limited access to drugs to long-term compulsive use of drugs. A dramatic escalation in drug intake with extended access to drug self-administration is characterized by a dysregulation of brain reward pathways. Hormonal studies using an experimenter-administered cocaine binge model and an escalation self-administration model have revealed large increases in ACTH and corticosterone in rats during an acute binge with attenuation during the chronic binge stage and a reactivation of the hypothalamic-pituitary-adrenal axis during acute withdrawal. The activation of the hypothalamic-pituitary-adrenal axis with cocaine appears to depend on feed-forward activation of the mesolimbic dopamine system. At the same time, escalation in drug intake with either extended access or dependence-induction produces an activation of the brain stress system's corticotropin-releasing factor outside of the hypothalamus in the extended amygdala, which is particularly evident during acute withdrawal. A model of the role of different levels of hormonal/brain stress activation in addiction is presented that has heuristic value for understanding individual vulnerability to drug dependence and novel treatments for the disorder.

Rationale, pharmacology and clinical efficacy of partial agonists of alpha4beta2 nACh receptors for smoking cessation

Most smokers repeatedly fail in their attempts to stop smoking because of the addictive nature of the nicotine in tobacco products. Nicotine dependence is probably mediated through the activation of multiple subtypes of neuronal nicotinic acetylcholine receptor (nAChR), among which the mesolimbic alpha(4)beta(2) subtype has a pivotal role. Here, we discuss the rationale for and the design of alpha(4)beta(2) nAChR partial agonists as novel treatments for tobacco addiction. Such agents are expected to exhibit a dual action by sufficiently stimulating alpha(4)beta(2)-nAChR-mediated dopamine release to reduce craving when quitting and by inhibiting nicotine reinforcement when smoking. Potent and selective alpha(4)beta(2) nAChR partial agonists that exhibit dual agonist and antagonist activity in preclinical models can be identified. The validity of this approach is demonstrated by the clinical efficacy of the alpha(4)beta(2) nAChR partial agonist varenicline, which has significantly better quit rates than do other treatments and offers a new option for smoking cessation pharmacotherapy.

Nornicotine inhibition of dopamine transporter function in striatum via nicotinic receptor activation

Nornicotine, a tobacco alkaloid and N-demethylated nicotine metabolite, releases DA from superfused rat striatal slices in a mecamylamine-sensitive manner, indicating nicotinic receptor (nAChR) modulation of this response. The current study determined the effect of nornicotine on rat striatal dopamine transporter (DAT) function using in vivo voltammetry. In a dose-related and mecamylamine-sensitive manner, nornicotine (0.35-12.0 mg/kg, s.c.) decreased DA clearance, suggesting that nornicotine inhibits striatal DAT function via a nAChR-mediated mechanism. Furthermore, the nAChRs mediating the nornicotine-induced inhibition of DAT function appear to be different from those activated by nicotine which increases DA clearance. Understanding the actions of nornicotine in brain may have significance for emerging therapeutics and for the management of nicotine dependence.

Nicotine and hippocampus-dependent learning: implications for addiction

Addiction is a complex disorder because many factors contribute to the development and maintenance of addiction. One factor is learning. For example, drug-context associations that develop during drug use could facilitate drug craving upon re-exposure to contexts previously associated with drugs. Additionally, deficits in cognitive processes associated with withdrawal could precipitate relapse in attempts to ameliorate those deficits. Because addiction and learning involve common neural areas and cell signaling cascades, addiction-related changes in processes underlying plasticity may contribute to addiction. This article examines similarities between addiction and learning at the behavioral, neural, and cellular levels, with emphasis on the neural substrates underlying the effects of acute nicotine, chronic nicotine, and withdrawal from chronic nicotine on hippocampus-dependent contextual learning.

Mecamylamine attenuates cue-induced reinstatement of nicotine-seeking behavior in rats

Mecamylamine, a noncompetitive nicotinic cholinergic antagonist, inhibits nicotine self-administration in animals and may attenuate tobacco smoking in humans trying to quit. Our preliminary data suggested that this agent, at a dose of 2 mg/kg (subcutaneous (s.c.)), also attenuates cue-induced relapse to nicotine-seeking behavior in rats. This study determined whether mecamylamine-induced attenuation can be obtained at doses lower than the high 2 mg/kg dose used in the first study, and whether it is specific to nicotine-associated cues. Male Sprague-Dawley rats were trained to intravenously self-administer nicotine (0.03 mg/kg/infusion) on a fixed-ratio 5 schedule. Each infusion was accompanied by a visual cue (1 s onset of a lever light followed by offset of a house light for 20 s during which time no infusions could be obtained). After the nicotine-maintained responding was extinguished by withholding the delivery of nicotine (saline substitution) and its associated cue, reinstatement tests were conducted. Response-contingent re-presentation of the cue without further availability of nicotine significantly reinstated extinguished responding at the previously nicotine-reinforced lever. Pretreatment with mecamylamine (0.5, 1, and 2 mg/kg, s.c.) dose-dependently attenuated the cue-induced reinstatement of lever responding. Mecamylamine did not change food-taking and -seeking responses, whereas the highest dose (2 mg/kg) decreased nicotine self-administration behavior. The results confirm previous findings that stimuli conditioned to nicotine self-administration effectively elicit reinstatement of nicotine-seeking behavior after extinction and demonstrate that mecamylamine, besides suppressing self-administration of nicotine, effectively attenuates cue-induced nicotine-seeking behavior. These findings suggest that the response-reinstatement procedures used in this study may be useful for studying neurobiological mechanisms of nicotine-seeking behavior and that mecamylamine-like drugs may be potential candidates for pharmacological treatment and prevention of relapse to tobacco smoking in abstinent smokers.

Smoking Cessation Pharmacotherapy?Nicotine and Non-Nicotine Preparations

Cigarette smokers tend to die prematurely from a number of diseases. Cigarette smoking is an important modifiable risk factor for cardiovascular morbidity and mortality. Despite the clear health benefits of smoking cessation, smokers usually find it difficult to stop and behavioral therapies often prove insufficient. Pharmacologic intervention may aid the process because of the addictive nature of nicotine. Nicotine replacement therapy, which is regarded as first-line therapy, was developed to overcome the symptoms of nicotine withdrawal that many patients find distressing. Different modes of administration include inhalation and buccal or transdermal absorption. The orally administered non-nicotine drugs varenicline and bupropion are also regarded as first-line treatments, either used alone or as an adjunct to nicotine replacement therapy. Second-line treatments include clonidine and nortriptyline. Other treatment strategies that have been examined include monoamine oxidase inhibitors and selective serotonin reuptake inhibitors; efficacy has yet to be proven definitively. A novel approach to treatment using the cannabinoid-1 receptor antagonist rimonabant is also under investigation.

Daily smoking patterns, their determinants, and implications for quitting

In this article, the authors examine daily temporal patterns of smoking in relation to environmental restrictions on smoking and cessation outcomes. Time-series methods were used for analyzing cycles in 351 smokers who monitored their smoking in real time for 2 weeks. The waking day was divided into 8 "bins" of approximately 2 hr, cigarette counts were tallied for each bin, and temporal patterns of smoking and restriction were analyzed. Cluster analyses of smoking patterns by time of day resulted in 4 clusters: daily decline (n = 30; 9%), morning high (n = 43; 12%), flatline (n = 247; 70%), and daily dip-evening incline (n = 31; 9%). Clusters differed in baseline demographic, smoking, and psychosocial variables. Results suggest that smoking behavior can be characterized by regular patterns of smoking frequency during the waking day: Smoking in the flatline cluster was within +/-0.5 standard deviation at all times. For the other clusters, smoking was high in the morning (daily dip-evening incline: +1.7 standard deviations; morning high: +2.8 standard deviations; daily decline: +1.7 standard deviations); moderate (morning high: -0.8 standard deviations; daily decline: +0.3 standard deviations) or low (daily dip-evening incline: -1.0 standard deviations) midday; and high (daily dip-evening incline: +2.0 standard deviations), moderate (morning high: +0.5 standard deviations), or low (daily decline: -1.5 standard deviations) in the evening. Daily smoking patterns were related to environmental smoking restrictions, but the strength of this relationship differed among clusters and by time of day. Clusters differed in lapse risk.

Pharmacological profile of the alpha4beta2 nicotinic acetylcholine receptor partial agonist varenicline, an effective smoking cessation aid

The preclinical pharmacology of the alpha4beta2 nicotinic acetylcholine receptor (nAChR) partial agonist varenicline, a novel smoking cessation agent is described. Varenicline binds with subnanomolar affinity only to alpha4beta2 nAChRs and in vitro functional patch clamp studies in HEK cells expressing nAChRs show that varenicline is a partial agonist with 45% of nicotine's maximal efficacy at alpha4beta2 nAChRs. In neurochemical models varenicline has significantly lower (40-60%) efficacy than nicotine in stimulating [(3)H]-dopamine release from rat brain slices in vitro and in increasing dopamine release from rat nucleus accumbens in vivo, while it is more potent than nicotine. In addition, when combined with nicotine, varenicline effectively attenuates the nicotine-induced dopamine release to the level of the effect of varenicline alone, consistent with partial agonism. Finally, varenicline reduces nicotine self-administration in rats and supports lower self-administration break points than nicotine. These data suggest that varenicline can reproduce to some extent the subjective effects of smoking by partially activating alpha4beta2 nAChRs, while preventing full activation of these receptors by nicotine. Based on these findings, varenicline was advanced into clinical development and recently shown to be an effective and safe aid for smoking cessation treatment.

Extensive overlap of mu-opioid and nicotinic sensitivity in cortical interneurons

We studied mu-opioid transmission in acute slices of rat neocortex using whole-cell recordings and single-cell reverse transcription-polymerase chain reaction. The mu-opioid receptor (MOR) was found in gamma-aminobutyric acidergic (GABAergic) interneurons that were either layer I cells frequently expressing neuropeptide Y or layers II-V cells expressing vasoactive intestinal peptide and enkephalin (Enk). We found that mu-opioid agonists inhibit these interneurons that are selectively excited by nicotinic agonists. The extensive overlap of mu-opioid and nicotinic responsiveness allowed mu-opioid agonists to inhibit nicotinic excitation of responsive interneurons and of their GABAergic output onto pyramidal cells. Finally, nicotinic stimulation resulted in a dynamic sequence where GABAergic transmission was first enhanced and then depressed below its baseline. This latter disinhibitory effect was prevented by a mu-opioid antagonist, indicating that excitation of nicotinic-responsive interneurons induced the release of endogenous Enk, which in turn led to MOR activation. Our results suggest that neocortical mu-opioid transmission acts as an inhibitory feedback onto nicotinic-responsive interneurons, which may change network excitability and inhibition patterns during cholinergic excitation.

Immediate hedonic response to smoking lapses: relationship to smoking relapse, and effects of nicotine replacement therapy

OBJECTIVE AND RATIONALE

Smoking lapses represent an important juncture between smoking cessation and relapse. Nicotine replacement therapy (NRT) has been shown to decrease the risk of progression from lapse to relapse. We hypothesized that this effect might be mediated via decreases in reinforcement from smoking lapses.

METHOD

We assessed 169 subjects who lapsed during treatment in a double-blind placebo-controlled study of high-dose (35 mg) nicotine patch. Following their first lapse, using an electronic diary, subjects recorded the amount they smoked, and rated the pleasantness and satisfaction ("hedonic rating") and the aversiveness of smoking. Subjects were then followed and assessed for further lapses and relapses.

RESULTS

Subjects who smoked more during the first lapse had greater risk of progression [second lapse: hazard ratio (HR)=1.16, confidence interval (CI)=1.01-1.32; relapse: HR=1.22, CI=0.97-1.54]. Subjects with higher hedonic ratings of the first lapse also had a greater risk of progression to the second lapse (HR=1.08, CI=1.02-1.14) and to relapse (HR=1.26, CI=1.11-1.41). Aversive ratings had no bearing on progression. As expected, active treatment reduced the risk of both a second lapse (HR=0.54, CI=0.39-0.78) and a relapse (HR=0.22, CI=0.11-0.45). Importantly, however, NRT had no effect on hedonic ratings, amount smoked during the first lapse, or aversive ratings.

CONCLUSIONS

Hedonic response to an initial lapse predicted progression to relapse, but this did not mediate the effect of NRT on progression.

Discrete opioid gene expression impairment in the human fetal brain associated with maternal marijuana use

Fetal development is a period sensitive to environmental influences such as maternal drug use. The most commonly used illicit drug by pregnant women is marijuana. The present study investigated the effects of in utero marijuana exposure on expression levels of opioid-related genes in the human fetal forebrain in light of the strong interaction between the cannabinoid and opioid systems. The study group consisted of 42 midgestation fetuses from saline-induced voluntary abortions. The opioid peptide precursors (preprodynorphin and preproenkephalin (PENK)) and receptor (mu, kappa and delta) mRNA expression were assessed in distinct brain regions. The effect of prenatal cannabis exposure was analyzed by multiple regression controlling for confounding variables (maternal alcohol and cigarette use, fetal age, sex, growth measure and post-mortem interval). Prenatal cannabis exposure was significantly associated with increased mu receptor expression in the amygdala, reduced kappa receptor mRNA in mediodorsal thalamic nucleus and reduced preproenkephalin expression in the caudal putamen. Prenatal alcohol exposure primarily influenced the kappa receptor mRNA with reduced levels in the amygdala, claustrum, putamen and insula cortex. No significant effect of prenatal nicotine exposure could be discerned in the present study group. These results indicate that maternal cannabis and alcohol exposure during pregnancy differentially impair opioid-related genes in distinct brain circuits that may have long-term effects on cognitive and emotional behaviors.

Genetic Influences on Smoking

Smoking is a complex behavior influenced by environ-mental and genetic factors. A number of twin studies demonstrate genetic influences on multiple aspects of smoking including initiation, dependence, continuation, amount consumed, and cessation. It is likely that this genetic risk involves a complex interaction between multiple genes in different biologic pathways. These may include genes involved in the general aspect of dependence (eg, genes within the dopaminergic system) and those more specific to nicotine dependence (eg, genes encoding nicotinic acetylcholine receptors and nicotine-metabolizing enzymes). Genome-wide linkage analyses and gene association studies provide approaches for identifying these genes. This review provides a brief highlight of data from some of the candidate gene association studies and discusses some of the discrepancies found in the literature. In addition, the authors have reviewed some recent studies examining how genetic variations in drug-metabolizing enzymes and drug targets can influence the response to treatment with drugs such as bupropion and nicotine.These latter studies provide the potential for maximizing treatment efficacy by individualizing the type, dosage, and duration of the treatment based on an individual's genotype/phenotype. Significant advances have been made in understanding the genetic contribution to smoking behaviors. These and future studies will provide potential directions for novel prevention and treatment strategies.

Wistar Kyoto rats exhibit reduced sucrose pellet reinforcement behavior and intravenous nicotine self-administration

A phenotype of heightened anxiety-like behavior is hypothesized to be associated with altered reinforcement behavior. To test this hypothesis, we studied patterns of sucrose pellet intake and intravenous nicotine self-administration in animals that exhibit anxiety-like behavior at baseline, Wistar Kyoto (WKY) rats, as compared to normal controls (Wistar rats). WKY rats exhibited significantly reduced sucrose pellet self-administration behavior as assessed by both fixed and progressive ratio schedules of reinforcement and exhibited significantly reduced self-administration of intravenous nicotine. On the basis of previously published findings, we hypothesize that altered mesolimbic dopamine responses, as well as heightened HPA axis functioning, may account for reduced nicotine self-administration and sucrose pellet reinforcement responding in WKY rats. These studies highlight the role of heightened anxiety-like behavior, resulting from the genetic background of the animal, in altering behavioral responses to reinforcing stimuli.

Withdrawal from chronic nicotine administration impairs contextual fear conditioning in C57BL/6 mice

The effects of acute nicotine administration (0.09 mg/kg nicotine), chronic nicotine administration (6.3 mg/kg/d nicotine for 14 d), and withdrawal from chronic nicotine administration on fear conditioning in C57BL/6 mice were examined. Mice were trained using two coterminating conditioned stimulus (30 s; 85 dB white noise)--unconditioned stimulus (2 s; 0.57 mA foot shock) pairings and tested 24 h later for contextual and cued fear conditioning. Acute nicotine administration enhanced contextual fear conditioning, chronic nicotine administration had no effect on contextual fear conditioning, and withdrawal from chronic nicotine administration impaired contextual fear conditioning. Plasma nicotine concentrations were similar after acute and chronic treatment and were within the range reported for smokers. During withdrawal, concentrations of nicotine were undetectable. An acute dose of nicotine (0.09 mg/kg) during withdrawal from chronic nicotine treatment reversed withdrawal-associated deficits in contextual fear conditioning. The results suggest that tolerance to the effects of nicotine on contextual fear conditioning develops with chronic nicotine treatment at a physiologically relevant dose, and withdrawal from this chronic nicotine treatment is associated with impairments in contextual fear conditioning. These findings provide a model of how the effects of nicotine on learning may contribute to the development and maintenance of nicotine addiction.

Involvement of the opioid system in the effects induced by nicotine on anxiety-like behaviour in mice

RATIONALE

Recent studies have revealed the participation of the endogenous opioid system in several behavioural responses induced by nicotine including antinociception, rewarding properties, and physical drug dependence.

OBJECTIVES

The present study was designed to examine the possible involvement of the various opioid receptors in the anxiolytic- and anxiogenic-like responses induced by nicotine in mice.

METHODS

The acute administration of low (0.05) or high (0.8 mg/kg) doses of nicotine subcutaneously produced opposite effects in the elevated plus maze, i.e. anxiolytic- and anxiogenic-like responses, respectively. Animals were only exposed once to nicotine. The effects of the pretreatment with the mu-opioid receptor antagonist, beta-funaltrexamine (5 mg/kg), the delta-opioid antagonist, naltrindole (2.5 mg/kg) and the kappa-opioid antagonist, nor-binaltorphimine (2.5 mg/kg) intraperitoneally were evaluated on the anxiolytic- and anxiogenic-like responses induced by nicotine.

RESULTS

beta-funaltrexamine, but not nor-binaltorphimine or naltrindole, abolished nicotine-induced anxiolytic-like effects, suggesting an involvement of mu-opioid receptors in this behavioural response. On the other hand, naltrindole, but not nor-binaltorphimine or beta-funaltrexamine, increased the anxiogenic-like responses of nicotine, suggesting an involvement of delta-receptors in this behavioural effect.

CONCLUSIONS

These results demonstrate that the endogenous opioid system is involved in the effects induced by nicotine on anxiety-like behaviour and provide new findings to further clarify the interaction between these two neurochemical systems.

Introduction of unsaturation into the N-n-alkyl chain of the nicotinic receptor antagonists, NONI and NDNI: effect on affinity and selectivity

N-n-octylnicotinium iodide (NONI) and N-n-decylnicotinium iodide (NDNI) are selective nicotinic receptor (nAChR) antagonists mediating nicotine-evoked striatal dopamine (DA) release, and inhibiting [3H]nicotine binding, respectively. This study evaluated effects of introducing unsaturation into the N-n-alkyl chains of NONI and NDNI on inhibition of [3H]nicotine and [3H]methyllycaconitine binding (alpha4beta2* and alpha7* nAChRs, respectively), (86)Rb+ efflux and [3H]DA release (agonist or antagonist effects at alpha4beta2* and alpha6beta2*-containing nAChRs, respectively). In the NONI series, introduction of a C3-cis- (NONB3c), C3-trans- (NONB3t), C7-double-bond (NONB7e), or C3-triple-bond (NONB3y) afforded a 4-fold to 250-fold increased affinity for [3H]nicotine binding sites compared with NONI. NONB7e and NONB3y inhibited nicotine-evoked 86Rb+ efflux, indicating alpha4beta2* antagonism. NONI analogs exhibited a 3-fold to 8-fold greater potency inhibiting nicotine-evoked [3H]DA overflow compared with NONI (IC50 = 0.62 microM; Imax = 89%), with no change in Imax, except for NONB3y (Imax = 50%). In the NDNI series, introduction of a C4-cis- (NDNB4c), C4-trans-double-bond (NDNB4t), or C3-triple-bond (NDNB3y) afforded a 4-fold to 80-fold decreased affinity for [3H]nicotine binding sites compared with NDNI, whereas introduction of a C9 double-bond (NDNB9e) did not alter affinity. NDNB3y and NDNB4t inhibited nicotine-evoked 86Rb+ efflux, indicating antagonism at alpha4beta2* nAChRs. Although NDNI had no effect, NDNB4t and NDNB9e potently inhibited nicotine-evoked [3H]DA overflow (IC50 = 0.02-0.14 microM, Imax = 90%), as did NDNB4c (IC50 = 0.08 microM; Imax = 50%), whereas NDNB3y showed no inhibition. None of the analogs had significant affinity for alpha7* nAChRs. Thus, unsaturated NONI analogs had enhanced affinity at alpha4beta2*- and alpha6beta2*-containing nAChRs, however a general reduction of affinity at alpha4beta2* and an uncovering of antagonist effects at alpha6beta2*-containing nAChRs were observed with unsaturated NDNI analogs.

Nicotine stimulates dendritic arborization in motor cortex and improves concurrent motor skill but impairs subsequent motor learning

The effect of the premature commitment of neurons to exuberant growth by nicotine on concurrent and subsequent learning is unknown and was the focus of the present study. Animals were trained on a tray reaching for food task (where lots of pieces of chicken feed were available) for 3 weeks before they received two daily injections of nicotine (0.3 mg/kg) or 0.9% saline for 12 days. Measures of tray-reaching performance were obtained before the administration of nicotine and every other week for a total of 7 weeks. Starting on week 8, animals were given a novel motor skill problem that required them to learn to use a forepaw to reach through a slot in a cage for single food pellets located on an external shelf. Pyramidal cells in the forelimb area of both hemispheres were then examined for dendritic length and branching using a Golgi-Cox procedure. Animals treated with saline displayed excellent performance in both reaching tasks and an increase in neuronal branching in Layer V pyramidal cells in the motor cortex contralateral to the reaching paw. In contrast, animals treated with nicotine showed bilateral increases in neuronal branching. Behavioral results showed that nicotine improved forelimb use in the concurrently administered tray-reaching task, but severely degraded quantitative and qualitative scores of skilled forelimb use in the subsequently administered single-pellet reaching task. The results suggest that plasticity coincidence with skilled training is essential to skilled motor learning, but this expenditure can impair subsequent learning.

Investigations using immunization to attenuate the psychoactive effects of nicotine

Despite the enormous health risks, people continue to smoke and use tobacco primarily as a result of nicotine addiction. As part of our immunopharmacotherapy research, the effects of active and passive immunizations on acute nicotine-induced locomotor activity in rats were investigated. To this end, rats were immunized with either a NIC-KLH immunoconjugate vaccine designed to elicit an antinicotine immune response, or were administered an antinicotine monoclonal antibody, NIC9D9, prior to a series of nicotine challenges and testing sessions. Vaccinated rats showed a 45% decrease in locomotor activity compared to a 16% decrease in controls. Passive immunization with NIC9D9 resulted in a 66.9% decrease in locomotor activity versus a 3.4% decrease in controls. Consistent with the behavioral data, much less nicotine was found in the brains of immunized rats. The results support the potential clinical value of immunopharmacotherapy for nicotine addiction in the context of tobacco cessation programs.

Selective changes in nicotinic acetylcholine receptor subtypes related to tobacco smoking: an immunohistochemical study

Increases in neuronal nicotinic receptors (nAChRs) in response to nicotine exposure have been reported in cell cultures, rodent brains, and in the brains of human smokers. The present study examines alterations in alpha4 and alpha7 nAChR subunit cellular expression in human hippocampus and entorhinal cortex from normal elderly individuals with known smoking history. There were significant increases in the intensity of alpha4 immunoreactive neuropil, but not the number of cell bodies, in many regions of hippocampus and entorhinal cortex in smokers compared to age-matched non-smokers and ex-smokers. There was also an increase in alpha7 immunoreactive perikarya in the granular cell layer of dentate gyrus in smokers but not other regions examined. There was, in contrast, a significant reduction in alpha7 immunoreactive astrocytes in smokers and ex-smokers compared to non-smokers. These findings suggest exposure to tobacco smoke acutely up-regulates alpha4 receptors in axon terminals and dendrites but not perikarya, whereas tobacco smoking induced down-regulation of alpha7 expression on astrocytes is a long-term effect. As the alpha4 subunit decreases with ageing and degenerative diseases such as Alzheimer's disease, whereas alpha7 increases in astrocytes in Alzheimer's disease, the findings further indicate the therapeutic relevance of nicotinic agonists such as nicotine.

A common haplotype of the nicotine acetylcholine receptor alpha 4 subunit gene is associated with vulnerability to nicotine addiction in men

Nicotine is the major addictive substance in cigarettes, and genes involved in sensing nicotine are logical candidates for vulnerability to nicotine addiction. We studied six single-nucleotide polymorphisms (SNPs) in the CHRNA4 gene and four SNPs in the CHRNB2 gene with respect to nicotine dependence in a collection of 901 subjects (815 siblings and 86 parents) from 222 nuclear families with multiple nicotine-addicted siblings. The subjects were assessed for addiction by both the Fagerstrom Test for Nicotine Dependence (FTND) and the Revised Tolerance Questionnaire (RTQ). Because only 5.8% of female offspring were smokers, only male subjects were included in the final analyses (621 men from 206 families). Univariate (single-marker) family-based association tests (FBATs) demonstrated that variant alleles at two SNPs, rs1044396 and rs1044397, in exon 5 of the CHRNA4 gene were significantly associated with a protective effect against nicotine addiction as either a dichotomized trait or a quantitative phenotype (i.e., age-adjusted FTND and RTQ scores), which was consistent with the results of the global haplotype FBAT. Furthermore, the haplotype-specific FBAT showed a common (22.5%) CHRNA4 haplotype, GCTATA, which was significantly associated with both a protective effect against nicotine addiction as a dichotomized trait (Z=-3.04, P<.005) and significant decreases of age-adjusted FTND (Z=-3.31, P<.005) or RTQ scores (Z=-2.73, P=.006). Our findings provide strong evidence suggesting a common CHRNA4 haplotype might be protective against vulnerability to nicotine addiction in men.

The impact of stress on addiction

This article will review data obtained from both clinical and preclinical investigations demonstrating that exposure to stress has a significant impact on drug addiction. The preclinical literature suggests that stress increases reward associated with psychomotor stimulants, possibly through a process similar to sensitization. While it is not conclusive that a similar process occurs in humans, a growing clinical literature indicates that there is a link between substance abuse and stress. One explanation for the high concordance between stress-related disorders and drug addiction is the self-medication hypothesis, which suggests that a dually diagnosed person often uses the abused substance to cope with tension associated with life stressors or to relieve symptoms of anxiety and depression resulting from a traumatic event. However, another characteristic of self-administration is that drug delivery and its subsequent effects on the hypothalamo-pituitary-adrenal (HPA) axis are under the direct control of the individual. This controlled activation of the HPA axis may result in the production of an internal state of arousal or stimulation that is actually sought by the individual (i.e., the sensation-seeking hypothesis). During abstinence, exposure to stressors or drug-associated cues can stimulate the HPA axis to remind the individual about the effects of the abused substance, thus producing craving and promoting relapse. Continued investigations into how stress and the subsequent activation of the HPA axis impact addiction will result in the identification of more effective and efficient treatment for substance abuse in humans. Stress reduction, either alone or in combination with pharmacotherapies targeting the HPA axis may prove beneficial in reducing cravings and promoting abstinence in individuals seeking treatment for addiction.

Effects of continuous nicotine infusion on nicotine self-administration in rats: relationship between continuously infused and self-administered nicotine doses and serum concentrations

RATIONALE

The efficacy of nicotine replacement therapy (NRT) for smoking cessation is limited. One reason for this limited efficacy may be that typical serum nicotine concentrations provided by NRT do not match the peak arterial nicotine concentrations achieved from smoking.

OBJECTIVE

The purpose of the present study was to determine whether continuous nicotine infusion at a rate producing serum nicotine concentrations that match the estimated peak arterial nicotine concentrations associated with nicotine self-administration (NSA) in rats produces greater suppression of NSA than lower infusion rates.

METHODS

The effects of continuous nicotine infusion were studied by intravenously administering nicotine at various rates (1.0, 3.0, and 8.0 mg/kg per day) to rats concurrently self-administering nicotine (0.03 mg/kg per infusion) during 23-h sessions or cocaine (0.17 mg/kg per infusion) during 2-h sessions.

RESULTS

Continuous nicotine infusion suppressed NSA in a rate-related fashion. NSA was suppressed by 17, 50, and 73% at infusion rates of 1.0, 3.0 and 8.0 mg/kg per day, respectively. The 8.0-mg/kg per day infusion rate, which provided venous serum nicotine concentrations equaling the peak arterial concentrations associated with NSA, suppressed NSA to a greater extent than lower infusion rates. The 8.0-mg/kg per day nicotine infusion rate had no effect on cocaine-maintained responding, demonstrating that its effects were specific for suppression of NSA. This infusion rate provided a mean percentage replacement of nicotine from NSA of more than 700%. Reacquisition of NSA after suppression by the two highest infusion rates was delayed compared with reacquisition after saline extinction.

CONCLUSIONS

Continuous nicotine infusion produced an infusion rate-related suppression of NSA that was greatest when the infusion provided nicotine doses and venous serum concentrations substantially higher than those typically associated with NRT in humans.

Nicotine and nicotinic receptor antagonists potentiate the antidepressant-like effects of imipramine and citalopram

1. Epidemiological and clinical observations suggest the involvement of nicotinic acetylcholine receptors (nAChRs) in depressive illness. Nonetheless, there is no clearcut evidence that nicotine and/or nAChR antagonists produce an antidepressant effect. 2. In the tail-suspension test (C57/Bl male mice), nicotine (0.8-1.2 mg kg(-1) s.c. or i.p.) given 15-60 min before the measurement exerted no effect on immobility. 3. Given 30 min before the measurement, citalopram (2 mg kg(-1)) produced a slight decrease in immobility; coadministration of nicotine (0.8 mg kg(-1), 15 but not 40 min before the test) to citalopram-treated mice resulted in a robust decrease in immobility. Imipramine (4 mg kg(-1)) did not affect immobility, but given in combination with 0.8 mg kg(-1) of nicotine (15 but not 40 min before the test), a decrease in immobility was observed. Nicotine (0.8 and 1.2 mg kg(-1)) also produced an enhancement in the anti-immobility effect of imipramine (20 mg kg(-1)). 4. We further investigated if nAChR antagonists would influence the antidepressant-like effects of imipramine and citalopram. Unexpectedly, mecamylamine (1-2.5 mg kg(-1)) and dihydro-beta-erythroidine (2 mg kg(-1)) potentiated the antidepressant-like effect of imipramine (4-20 mg kg(-1)). Mecamylamine (2.5 mg kg(-1)) but not dihydro-beta-erythroidine also increased the antidepressant-like effect produced by 2 mg kg(-1) of citalopram. 5. The interaction between nAChR antagonists and antidepressants appeared synergistic. 6. Neither nAChR ligands, antidepressants nor combinations of the two, affected locomotor activity. 7. The present results demonstrate an unexpected interaction between nAChR ligands and imipramine and citalopram in the tail-suspension test.

Pharmacotherapy of addictions

Addiction to drugs, such as heroin, cocaine and alcohol, exacts great human and financial costs on society, but the development of pharmacotherapies for addiction has been largely neglected by the pharmaceutical industry. With advances in our understanding of the underlying biology of addictions now opening the door for the development of novel pharmacotherapies, it could be time for a reassessment of involvement in this increasingly important therapeutic area. Here, we summarize the current approved and implemented pharmacotherapeutic approaches to the treatment of addiction, and then highlight the most promising areas for future drug development from the perspective of our laboratory and our National Institutes of Health (NIH) National Institute on Drug Abuse (NIDA) Research Center.