Morphine-nicotine interaction in conditioned place preference in mice after chronic nicotine exposure

Modulators of nicotine reward and reinforcement

Nicotine has been well-characterized for its ability to alter neurophysiology to promote rewarding and reinforcing properties. However, several exogenous chemicals possess properties that modulate or enhance nicotine's ability to alter neurophysiology. This chapter focuses on nicotine's impact on behavior through changes in neurophysiology and several chemical entities that in-turn modulate nicotine's ability to act as a neuromodulator.

Incidence of opioid misuse by cigarette smoking status in the United States

BACKGROUND

The combination of opioid misuse and cigarette smoking contributes to increased morbidity and mortality compared to each substance use alone. We estimated the incidence of opioid misuse for persons who currently or formerly smoked versus never smoked.

METHODS

Data came from the 2015-2020 National Surveys on Drug Use and Health cross-sectional surveys of US civilians aged 12+ (n = 315,661). Weighted opioid misuse incidence and average time between cigarette use initiation and opioid misuse initiation were calculated annually by smoking status. Logistic regression models tested time trends in incidence by smoking status.

RESULTS

Overall, 0.75% of persons initiated opioid misuse per year; opioid misuse incidence was 1.35% for those who currently smoked cigarettes, 0.54% for those who formerly smoked, and 0.67% for those who never smoked. For persons who currently smoked and misused opioids (1.50%), 95.08% smoked prior to opioid use. The average time between smoking followed by opioid misuse was 12.93 years and for opioid misuse followed by smoking was 4.36 years. Persons who currently smoked were more likely to initiate opioid misuse than those who had never smoked (AOR = 1.81, 95% CI: 1.60, 2.06). There was a decrease in the opioid misuse incidence over time (AOR = 0.90; 95% CI: 0.85, 0.92), which did not differ by smoking status.

CONCLUSIONS

Persons who currently smoked cigarettes, relative to those who never smoked, were more likely to initiate opioid misuse. As most individuals smoked before opioid misuse, it may be useful for primary prevention efforts to decrease opioid misuse initiation by focusing on smoking status.

A common SNP in Chrna5 enhances morphine reward in female mice

The single nucleotide polymorphism (SNP) D398N (rs16969968) in CHRNA5, the gene encoding the α5 subunit of the nicotinic acetylcholine receptors (nAChR), has been associated with both nicotine and opiate dependence in human populations. Expression of this SNP on presynaptic VTA dopaminergic (DA) neurons is known to cause a reduction in calcium signaling, leading to alterations in transmitter signaling and altered responses to drugs of abuse. To examine the impact of the Chrna5 SNP on opiate reward and underlying dopaminergic mechanisms, mice harboring two copies of the risk-associated allele (Chrna5 A/A) at a location equivalent to human rs16969968 were generated via CRISPR/cas9 genome editing. We sought to determine whether Chrna5 A/A mice show differences in sensitivity to rewarding properties of morphine using the conditioned place preference paradigm. When mice were tested two weeks after conditioning, female Chrna5 A/A mice showed significantly enhanced preference for the morphine-paired chamber relative to WT females, suggesting that this genotype may enhance opioid reward specifically in females. In contrast, Chrna5 genotype had no effect on locomotor sensitization in male or female mice. Relative to WT females, peak amplitude of ACh-gated currents recorded from VTA DA neurons in Chrna5 A/A females was potentiated 1 day after conditioning with morphine. Increased FOS expression was also observed in Chrna5 A/A mice relative to WT mice following exposure to the morphine CPP chamber. We propose that impaired α5 nAChR subunit function alters DA neuron response following repeated morphine exposures, and that this early cellular response could contribute to enhanced opiate reward two weeks after conditioning.

Nicotine and opioid co-dependence: Findings from bench research to clinical trials

Concomitant use of tobacco and opioids represents a growing public health concern. In fact, the mortality rate due to smoking-related illness approaches 50% among SUD patients. Cumulative evidence demonstrates that the vulnerability to drugs of abuse is influenced by behavioral, environmental, and genetic factors. This review explores the contribution of genetics and neural mechanisms influencing nicotine and opioid reward, respiration, and antinociception, emphasizing the interaction of cholinergic and opioid receptor systems. Despite the substantial evidence demonstrating nicotine-opioid interactions within the brain and on behavior, the currently available pharmacotherapies targeting these systems have shown limited efficacy for smoking cessation on opioid-maintained smokers. Thus, further studies designed to identify novel targets modulating both nicotinic and opioid receptor systems may lead to more efficacious approaches for co-morbid nicotine dependence and opioid use disorder.

Adolescent cannabis and tobacco use are associated with opioid use in young adulthood-12-year longitudinal study in an urban cohort

BACKGROUND AND AIMS

Cannabis, tobacco and alcohol use are prevalent among youth in the United States and may be risk factors for opioid use. The current study aimed at investigating associations between developmental trajectories of cannabis, tobacco and alcohol use in adolescence and opioid use in young adulthood in an urban cohort over the span of 12 years.

DESIGN

Cohort study of adolescents originally recruited for a randomized prevention trial with yearly assessments into young adulthood.

SETTING

Nine urban elementary schools in Baltimore, MD in the United States.

PARTICIPANTS

Participants (n = 583, 86.8% African American, 54.7% male) were originally recruited as first grade students.

MEASUREMENTS

Cannabis, tobacco and alcohol use were assessed annually from ages 14-18 years and opioid use from ages 19-26. Socio-demographics were assessed at age 6. Intervention status was also randomly assigned at age 6. Gender, race, free/reduced-priced lunch and intervention status were included as covariates in individual and sequential growth models.

FINDINGS

There were significant positive associations between the cannabis use intercept at age 14 and the opioid use intercept at age 19 (beta = 1.43; P = 0.028), the tobacco use intercept at age 14 and the opioid use intercept at age 19 (beta = 0.82; P = 0.042). Specifically, more frequent use of cannabis or tobacco at age 14 was associated with more frequent use of opioids at age 19.

CONCLUSIONS

Cannabis and tobacco use in early adolescence may be risk factors for opioid use in young adulthood among African Americans living in urban areas.

Early adolescent subchronic low-dose nicotine exposure increases subsequent cocaine and fentanyl self-administration in Sprague-Dawley rats

An exponential rise in nicotine-containing electronic-cigarette use has been observed during the period of adolescence. Preclinical studies have shown that nicotine exposure during early adolescence, but not adulthood, increases subsequent drug intake and reward. Although growing clinical trends highlight that stimulant use disorders are associated with the opioid epidemic, very few studies have assessed the effects of adolescent nicotine exposure on opioid intake. The objective of our current study is to develop a new animal model to assess the causal relationship of adolescent nicotine exposure on subsequent opioid intake. In this effort, we first replicate previous studies using a well-established 4-day nicotine paradigm. Rats are pretreated with a low dose of nicotine (2 × , 30 μg/kg/0.1 mL, intravenous) or saline during early adolescence (postnatal days 28-31) or adulthood (postnatal days 86-89). Following nicotine pretreatment on postnatal day 32 or postnatal day 90, animals underwent operant intravenous self-administration for the psychostimulant, cocaine [500 μg/kg/infusion (inf)] or the opioid, fentanyl (2.5 μg/kg/inf). We successfully show that adolescent but not adult, nicotine exposure enhances cocaine self-administration in male rats. Furthermore, we illustrate early adolescent but not adult nicotine exposure enhances fentanyl self-administration, independent of sex. Overall, our findings highlight that adolescence is a unique period of development that is vulnerable to nicotine-induced enhancement for cocaine and fentanyl self-administration in rats.

Independent association of tobacco use with opioid use disorder in patients of European ancestry with chronic non-cancer pain

BACKGROUND

The harms associated with prescription opioid abuse have become a public health crisis. There is a need for evidence-based objective markers of the risk of opioid use disorder (OUD) in patients with pain receiving opioid treatment. The objective of this study was to evaluate the independent association of tobacco use and OUD in patients with chronic non-cancer pain.

METHODS

This cross-sectional naturalistic study evaluated 798 adults ≥ 18 years with chronic non-cancer pain treated with long-term opioid therapy (≥ 6 months) who either developed an OUD (cases, n = 216) or displayed no evidence of an OUD (controls, n = 582). The primary outcome was presence of OUD. In addition to current self-reported tobacco use (primary predictor), covariates included demographics, pain severity, and psychiatric history. Data were collected between November 2012 and September 2018.

RESULTS

Current tobacco use independently was strongly associated with OUD [odds ratio (OR) 14.0, 95 % confidence interval (CI) 9.5-20.6, p < 0.001], and this association remained significant after adjusting for other risk factors [adjusted odds ratio (aOR) 7.6, 95 % CI 4.8-12.2, p < 0.001]. Other factors associated independently with development of OUD included age, marital status, financial status, education and pain severity.

CONCLUSIONS AND RELEVANCE

Current tobacco use is significantly associated with OUD in patients with chronic pain receiving long-term opioid therapy.

Acute Nicotine Exposure Alters Ventral Tegmental Area Inhibitory Transmission and Promotes Diazepam Consumption

Nicotine use increases the risk for subsequent abuse of other addictive drugs, but the biological basis underlying this risk remains largely unknown. Interactions between nicotine and other drugs of abuse may arise from nicotine-induced neural adaptations in the mesolimbic dopamine (DA) system, a common pathway for the reinforcing effects of many addictive substances. Previous work identified nicotine-induced neuroadaptations that alter inhibitory transmission in the ventral tegmental area (VTA). Here, we test whether nicotine-induced dysregulation of GABAergic signaling within the VTA increases the vulnerability for benzodiazepine abuse that has been reported in smokers. We demonstrate in rats that nicotine exposure dysregulates diazepam-induced inhibition of VTA GABA neurons and increases diazepam consumption. In VTA GABA neurons, nicotine impaired KCC2-mediated chloride extrusion, depolarized the GABAA reversal potential, and shifted the pharmacological effect of diazepam on GABA neurons from inhibition toward excitation. In parallel, nicotine-related alterations in GABA signaling observed ex vivo were associated with enhanced diazepam-induced inhibition of lateral VTA DA neurons in vivo Targeting KCC2 with the agonist CLP290 normalized diazepam-induced effects on VTA GABA transmission and reduced diazepam consumption following nicotine administration to the control level. Together, our results provide insights into midbrain circuit alterations resulting from nicotine exposure that contribute to the abuse of other drugs, such as benzodiazepines.

Interaction of nicotine with morphine potency in Paramecium caudatum

Aims

Many studies have been carried out about the interaction between nicotine and morphine in higher animals. Although previous behavioral and pharmacological evidence indicated the presence of opioid system in protozoa, there is no available data about nicotine effect on the potency of morphine in eukaryotic unicellular organisms such as protozoa. Hence, this work aims to investigate the interaction of nicotine with morphine in the protozoan Paramecium caudatum.

Main methods

According to our innovative model, the movement behavior of P. caudatum was investigated with a numerical scale using the Sedgewick-Rafter counting chamber at the field of view of 4X light microscope objective lens, such that the difference in number of Paramecia cells at definite moments after injection of drugs/substances was considered as a criterion for the behavioral response of P. caudatum.

Key findings

Results indicated the variations of morphine potency at the dose of 2 μg morphine accompanied by different doses of nicotine in P. caudatum so that the highest aggregation of Paramecia cells occurred at the dose of 2 μg morphine + 4 μg nicotine.

Significance

This confirmed that in eukaryotic unicellular organisms such as P. caudatum, nicotine can reinforce the morphine potency in a dose-dependent manner.

Tobacco Smoking and Brain Endogenous Opioid Release: More Than Nicotine Alone

INTRODUCTION

The effects of smoking denicotinized (denic) and average nicotine (avnic) tobacco cigarettes were studied on brain mu opioid receptor binding by positron emission tomography with 11C carfentanil. The results indicated the importance of physiological and psychological effects induced by denic smoking.

METHODS

Regional mu opioid binding potential (nondisplaceable binding potential, BPND) was measured in 20 adult male overnight abstinent chronic tobacco smokers. The denic sessions were conducted about 8:00 am followed by avnic sessions about 2 hours later. Venous plasma nicotine levels and scores of craving to smoke were assessed before and after each smoking session. Fagerstrom scores of nicotine dependence were determined. Pearson's and Spearman's correlation tests were used to examine associations between BPND and other smoking parameters.

RESULTS

Surprisingly, the very low plasma nicotine peak levels after denic smoking (mean ± SD: 3.3 ± 1.8 ng/mL) were significantly correlated with BPND after denic and avnic smoking. Equally surprising no association was found between nicotine levels after avnic smoking and BPND. Delta craving scores and Fagerstrom scores were correlated with both BPND after denic and avnic in several brain regions.

CONCLUSIONS

Very small amounts of nicotine, psychological and behavioral effects of denic smoking appear to have important actions on the endogenous mu opioid system.

IMPLICATIONS

Associations between very low venous plasma nicotine levels after denic smoking and regional brain mu opioid receptor availability are a surprising "placebo" effect. Delta craving and Fagerstrom scores were correlated with BPND in several brain regions including amygdala, hippocampus, insula, nucleus accumbens, putamen, and ventral striatum. This study is limited by modest Power (mean 1 - β = 0.6) for all correlation analyses.

Prevalence and patterns of opioid misuse and opioid use disorder among primary care patients who use tobacco

BACKGROUND

Current data suggest that opioid misuse or opioid use disorder (OUD) may be over represented among tobacco users. However, this association remains understudied in primary care settings. A better understanding of the extent of heterogeneity in opioid misuse among primary care patients who use tobacco may have implications for improved primary care-based screening, prevention, and intervention approaches.

METHODS

Data were derived from a sample of 2000 adult (aged ≥18) primary care patients across 5 distinct clinics. Among past-year tobacco users (n = 882), we assessed the prevalence of opioid misuse and OUD by sociodemographic characteristics and past-year polysubstance use. Latent class analysis (LCA) was used to identify heterogeneous subgroups of tobacco users according to past-year polysubstance use patterns. Multinomial logistic regression was used to examine variables associated with LCA-defined class membership.

RESULTS

Past-year tobacco use was reported by >84% of participants who reported past-year opioid misuse or OUD. Among those reporting past-year tobacco use, the prevalence of past-year opioid misuse and OUD was 14.0% and 9.5%, respectively. The prevalence of opioid misuse or OUD was highest among tobacco users who were male or unemployed. Three LCA-defined classes among tobacco users were identified including a tobacco-minimal drug use group (78.0%), a tobacco-cannabis use group (10.1%), and a tobacco-opioid/polydrug use group (11.9%). Class membership differed by sociodemographic characteristics.

CONCLUSIONS

Results from this study support the benefit of more comprehensive assessment of and/or monitoring for opioid misuse among primary care patients who use tobacco, particularly for those who are male, unemployed, or polydrug users.

Mechanisms and Clinical Features of Co-occurring Opioid and Nicotine Use

Purpose of review

To review the literature addressing shared pathophysiological and clinical features of opioid and nicotine use to inform etiology and treatment, and highlight areas for future research.

Recent findings

Opioid and nicotine use co-occur at an alarmingly high rate, and this may be driven in part by interactions between the opioid and cholinergic systems underlying drug reward and the transition to dependence. Pain, among other shared risk factors, is strongly implicated in both opioid and nicotine use and appears to play an important role in their co-occurrence. Additionally, there are important sex/gender considerations that require further study. Regarding treatment, smoking cessation can improve treatment outcomes in opioid use disorder, and pharmacological approaches that target the opioid and cholinergic systems may be effective for treating both classes of substance use disorders.

Summary

Understanding overlapping etiological and pathophysiological mechanisms of opioid and nicotine use can aid in understanding their co-occurrence and guiding their treatment.

Endogenous opioid system: a promising target for future smoking cessation medications

BACKGROUND

Nicotine addiction continues to be a health challenge across the world. Despite several approved medications, smokers continue to relapse. Several human and animal studies have evaluated the role of the endogenous opioid system as a potential target for smoking cessation medications.

METHODS

In this review, studies that have elucidated the role of the mu (MORs), delta (DORs), and kappa (KORs) opioid receptors in nicotine reward, nicotine withdrawal, and reinstatement of nicotine seeking will be discussed. Additionally, the review will discuss discrepancies in the literature and therapeutic potential of the endogenous opioid system, and suggest studies to address gaps in knowledge with respect to the role of the opioid receptors in nicotine dependence.

RESULTS

Data available till date suggest that blockade of the MORs and DORs decreased the rewarding effects of nicotine, while activation of the MORs and DORs decreased nicotine withdrawal-induced aversive effects. In contrast, activation of the KORs decreased the rewarding effects of nicotine, while blockade of the KORs decreased nicotine withdrawal-induced aversive effects. Interestingly, blockade of the MORs and KORs attenuated reinstatement of nicotine seeking. In humans, MOR antagonists have shown benefits in select subpopulations of smokers and further investigation is required to realize their full therapeutic potential.

CONCLUSION

Future work must assess the influence of polymorphisms in opioid receptor-linked genes in nicotine dependence, which will help in both identifying individuals vulnerable to nicotine addiction and the development of opioid-based smoking cessation medications. Overall, the endogenous opioid system continues to be a promising target for future smoking cessation medications.

Anxiety and Depression in Bidirectional Relations Between Pain and Smoking: Implications for Smoking Cessation

Pain and tobacco smoking are highly prevalent and comorbid conditions that impose considerable burdens on individuals and health care systems. A recently proposed reciprocal model suggests that these conditions interact in a bidirectional manner, resulting in greater pain and the maintenance of tobacco addiction. Anxiety and depression are common among smokers in pain and have been identified as central mechanisms of interest. There is emerging evidence that smokers with anxiety/depression may experience more severe pain and functional impairment, greater pain-induced motivation to smoke, and increased sensitivity to pain during periods of smoking abstinence. Based on empirical findings, we hypothesize that these experiences may engender expectations that abstaining from smoking will exacerbate both pain and negative affect, thus eroding self-efficacy for smoking cessation and increasing perceived barriers to quitting. The goal of this narrative review is to examine the role of anxiety/depression in complex pain-smoking relations so as to advance evolving theoretical perspectives and inform the development of tailored interventions.

The opioid receptors as targets for drug abuse medication

The endogenous opioid system is largely expressed in the brain, and both endogenous opioid peptides and receptors are present in areas associated with reward and motivation. It is well known that this endogenous system plays a key role in many aspects of addictive behaviours. The present review summarizes the modifications of the opioid system induced by chronic treatment with drugs of abuse reported in preclinical and clinical studies, as well as the action of opioid antagonists and agonists on the reinforcing effects of drugs of abuse, with therapeutic perspectives. We have focused on the effects of chronic psychostimulants, alcohol and nicotine exposure. Taken together, the changes in both opioid peptides and opioid receptors in different brain structures following acute or chronic exposure to these drugs of abuse clearly identify the opioid system as a potential target for the development of effective pharmacotherapy for the treatment of addiction and the prevention of relapse.

Nicotine Increases Codeine Analgesia Through the Induction of Brain CYP2D and Central Activation of Codeine to Morphine

CYP2D metabolically activates codeine to morphine, which is required for codeine analgesia. Permeability across the blood-brain barrier, and active efflux, suggests that initial morphine in the brain after codeine is due to brain CYP2D metabolism. Human CYP2D is higher in the brains, but not in the livers, of smokers and 7-day nicotine treatment induces rat brain, but not hepatic, CYP2D. The role of nicotine-induced rat brain CYP2D in the central metabolic activation of peripherally administered codeine and resulting analgesia was investigated. Rats received 7-day nicotine (1 mg/kg subcutaneously) and/or a single propranolol (CYP2D mechanism-based inhibitor; 20 μg intracerebroventricularly) pretreatment, and then were tested for analgesia and drug levels following codeine (20 mg/kg intraperitoneally) or morphine (3.5 mg/kg intraperitoneally), matched for peak analgesia. Nicotine increased codeine analgesia (1.59X AUC(0-30 min) vs vehicle; p<0.001), while propranolol decreased analgesia (0.56X; p<0.05); co-pretreatment was similar to vehicle controls (1.23X; p>0.1). Nicotine increased, while propranolol decreased, brain, but not plasma, morphine levels, and analgesia correlated with brain (p<0.02), but not plasma (p>0.4), morphine levels after codeine. Pretreatments did not alter baseline or morphine analgesia. Here we show that brain CYP2D alters drug response despite the presence of substantial first-pass metabolism of codeine and further that nicotine induction of brain CYP2D increases codeine response in vivo. Thus variation in brain CYP2D activity, due to genetics or environment, may contribute to individual differences in response to centrally acting substrates. Exposure to nicotine may increase central drug metabolism, not detected peripherally, contributing to altered drug efficacy, onset time, and/or abuse liability.

Tobacco Smoking, Nicotine Dependence, and Patterns of Prescription Opioid Misuse: Results From a Nationally Representative Sample

INTRODUCTION

The misuse of prescription opioid medications is a growing public health crisis. Given evidence of complex nicotine-opioid interactions, and initial support for the role of smoking status as a risk factor for prescription opioid misuse, a more detailed analysis of how current and historical patterns of smoking may influence misuse of prescription opioids is warranted.

METHODS

The current study is the first to test whether varying levels of current/historical smoking (current daily, current intermittent, former daily, never) and indices of smoking heaviness/nicotine dependence may be associated with greater likelihood of past-year prescription opioid misuse in the general population. Data were derived from the National Survey on Drug Use and Health (N = 24,348).

RESULTS

Consistent with hypotheses, after accounting for sociodemographic factors and major depressive/alcohol use disorders, both daily and intermittent smokers were greater than 3 times more likely to report past-year nonmedical prescription opioid use than were never smokers. In addition, daily smokers were observed to be nearly 5 times more likely, and intermittent smokers were nearly 3 times more likely, to have met past-year abuse/dependence criteria, relative to never smokers. Results further revealed positive associations between various indices of smoking heaviness/nicotine dependence and opioid medication misuse, and these findings remained largely consistent when analyses were stratified by gender.

CONCLUSIONS

These findings indicate that smokers are not a homogeneous group with regard to risk for opioid misuse, and support the utility of comprehensive smoking assessment in the context of opioid-based treatment/tapering.

Environmental enrichment improves novel object recognition and enhances agonistic behavior in male mice

Environmental enrichment (EE) is an experimental paradigm in which rodents are housed in complex environments containing objects that provide stimulation, the effects of which are expected to improve the welfare of these subjects. EE has been shown to considerably improve learning and memory in rodents. However, knowledge about the effects of EE on social interaction is generally limited and rather controversial. Thus, our aim was to evaluate both novel object recognition and agonistic behavior in NMRI mice receiving EE, hypothesizing enhanced cognition and slightly enhanced agonistic interaction upon EE rearing. During a 4-week period half the mice (n = 16) were exposed to EE and the other half (n = 16) remained in a standard environment (SE). On PND 56-57, animals performed the object recognition test, in which recognition memory was measured using a discrimination index. The social interaction test consisted of an encounter between an experimental animal and a standard opponent. Results indicated that EE mice explored the new object for longer periods than SE animals (P < .05). During social encounters, EE mice devoted more time to sociability and agonistic behavior (P < .05) than their non-EE counterparts. In conclusion, EE has been shown to improve object recognition and increase agonistic behavior in adolescent/early adulthood mice. In the future we intend to extend this study on a longitudinal basis in order to assess in more depth the effect of EE and the consistency of the above-mentioned observations in NMRI mice.

Morphine-induced anxiolytic-like effect in morphine-sensitized mice: involvement of ventral hippocampal nicotinic acetylcholine receptors

In the present study, the effects of repeated intra-ventral hippocampal (intra-VH) microinjections of nicotinic acetylcholine receptor agonist or antagonist on morphine-induced anxiolytic-like behavior were investigated in morphine-sensitized mice using elevated plus-maze. Intraperitoneal (i.p.) administration of different doses of morphine (5, 7.5 and 10mg/kg) increased the percentage of open arm time (%OAT), open arm entries (%OAE), but not locomotor activity, indicating an anxiolytic-like response to morphine. The maximum response was obtained by 7.5mg/kg of the opioid. The anxiety-like behavior which was induced by a lower dose of morphine (5mg/kg) was significantly increased in mice that had previously received once daily injections of morphine (10 and 20mg/kg, i.p.) for 3 days. It should be considered that this treatment also increased locomotor activity in morphine-sensitized mice. Furthermore, the response to an ineffective dose of morphine (5mg/kg, i.p.) in the EPM was significantly increased in the animals that had previously received nicotine for 3 days (0.1, 0.3, 0.5 and 0.7 μg/mouse; intra-VH), 5 min prior to the injections of morphine (5mg/kg/day × 3 days; i.p.). On the other hand, the increase of morphine-induced anxiolytic-like effect in animals that had previously received the 3-day morphine (20mg/kg) was dose dependently suppressed by once daily injections of mecamylamine (0.5, 1 and 2 μg/mouse/day × 3 days; intra-VH). It is important to note that repeated intra-VH administrations of the same doses of nicotine or mecamylamine alone caused no significant change in morphine (5mg/kg)-induced anxiety-like parameters in the EPM. In conclusion, it seems that morphine sensitization affects the anxiety-like behavior in the EPM and the cholinergic system in the ventral hippocampus, via nicotinic receptors, may play an important role in this effect.

A systematic review of the A118G (Asn40Asp) variant of OPRM1 in relation to smoking initiation, nicotine dependence and smoking cessation

Candidate gene studies on smoking behaviors mainly focused on dopaminergic and serotonergic genes, but genes within the µ-opioid system might also be involved. The A118G variant within the OPRM1 gene has been most often examined in relation to smoking, yielding inconsistent findings. It is largely unknown which of the alleles increases susceptibility for smoking behaviors. The aim of this review was to merge findings of OPRM1 gene studies in relation to smoking behaviors and to elaborate on the underlying biological mechanism of the A118G variant. It appeared that A118 was more likely to increase susceptibility to smoking behaviors than 118G, especially with regard to nicotine dependence, but less with smoking initiation and cessation. The proposed functioning of the OPRM1 gene is further explained.

Neurobiological mechanisms involved in nicotine dependence and reward: participation of the endogenous opioid system

Nicotine is the primary component of tobacco that maintains the smoking habit and develops addiction. The adaptive changes of nicotinic acetylcholine receptors produced by repeated exposure to nicotine play a crucial role in the establishment of dependence. However, other neurochemical systems also participate in the addictive effects of nicotine including glutamate, cannabinoids, GABA and opioids. This review will cover the involvement of these neurotransmitters in nicotine addictive properties, with a special emphasis on the endogenous opioid system. Thus, endogenous enkephalins and beta-endorphins acting on mu-opioid receptors are involved in nicotine-rewarding effects, whereas opioid peptides derived from prodynorphin participate in nicotine aversive responses. An up-regulation of mu-opioid receptors has been reported after chronic nicotine treatment that could counteract the development of nicotine tolerance, whereas the down-regulation induced on kappa-opioid receptors seems to facilitate nicotine tolerance. Endogenous enkephalins acting on mu-opioid receptors also play a role in the development of physical dependence to nicotine. In agreement with these actions of the endogenous opioid system, the opioid antagonist naltrexone has shown to be effective for smoking cessation in certain sub-populations of smokers.

Temporal changes of smoking status and motivation among Chinese heroin-dependent, methadone-maintained smokers

INTRODUCTION

The rates of cigarette smoking remain extremely high in active heroin users and methadone-maintained patients. It remains undetermined whether smoking status and motivation would be differentially affected by heroin and methadone administration.

METHODS

Heroin-dependent, methadone-maintained patients were recruited in the present studies. A battery of self-report questionnaires was used in the current study, in order to assess smoking status and motivations before first heroin use, during active heroin use and after Methadone Maintenance Treatment (MMT) admission.

RESULTS

An extremely high portion of participants started smoking before first heroin use. The highest level of cigarette smoking was found during the period of active heroin use, and cigarette consumption was reported to decrease after MMT admission. A wide range of smoking motivations were found before first heroin use. Moreover, "maintaining heroin pleasure" was the primary motivation for the increase in cigarette consumption during the period of active heroin use and 1h after heroin administration, while "habitual smoking" was the primary smoking motivation before first heroin use and after MMT admission respectively.

CONCLUSIONS

The present study first demonstrated that the prolonged rewarding effect of heroin following cigarette smoking may account for the increase of nicotine consumption found in the heroin-dependent patients. It appears that heroin and methadone differentially influenced smoking status and motivation among heroin-dependent, methadone-maintained patients.

Nicotine dependence predicts repeated use of prescribed opioids. Prospective population-based cohort study

PURPOSE

The aim of this study was to evaluate prospectively smoking dependence as a predictor of repeated use of prescribed opioids in non-cancer patients.

METHODS

We conducted a prospective population-based study cohort of 12,848 men and 15,894 women 30-75 years of age in health surveys in Norway during 2000-2002 with repeated opioid prescriptions (12+, during 2004-2007) recorded in the Norwegian Prescription Database as the outcome measure. Information on history of smoking and potential confounders was obtained at baseline by self-administered questionnaires. For smoking, participants were divided into categories: never; previously heavy (stopped maximum of 5 years earlier; 10+ cigarettes daily); daily not heavy (1-9 cigarettes); dependent daily smokers (10+ cigarettes), and other (previously and/or not daily). Odds ratios (ORs) with 95% confidence intervals (CIs) were estimated by logistic regression.

RESULTS

During follow-up, 335 (1.5%) of survey participants were registered with 12+ prescriptions of opioids during the period 2004-2007. The prevalence of repeated prescription frequency of opioids was higher for men and women with a history of smoking. The adjusted OR for prescribed opioids for dependent daily smokers was 3.1 (95% CI: 2.3-4.1), for daily non-heavy smokers 1.8 (1.2-2.7), and for previous heavy smokers 1.8 (1.1-3.0), compared with never-smokers as reference.

CONCLUSIONS

Results of the study suggest that smoking dependence may predict more frequent use of opioids.

Nicotine: alcohol reward interactions

It is well established that the continued intake of drugs of abuse is reinforcing-that is repeated consumption increases preference. This has been shown in some studies to extend to other drugs of abuse; use of one increases preference for another. In particular, the present review deals with the interaction of nicotine and alcohol as it has been shown that smoking is a risk factor for alcoholism and alcohol use is a risk factor to become a smoker. The review discusses changes in the brain caused by chronic nicotine and chronic alcohol intake to approach the possible mechanisms by which one drug increases the preference for another. Chronic nicotine administration was shown to affect nicotine receptors in the brain, affecting not only receptor levels and distribution, but also receptor subunit composition, thus affecting affinity to nicotine. Other receptor systems are also affected among others catecholamine, glutamate, GABA levels and opiate and cannabinoid receptors. In addition to receptor systems and transmitters, there are endocrine, metabolic and neuropeptide changes as well induced by nicotine. Similarly chronic alcohol intake results in changes in the brain, in multiple receptors, transmitters and peptides as discussed in this overview and also illustrated in the tables. The changes are sex and age-dependent-some changes in males are different from those in females and in general adolescents are more sensitive to drug effects than adults. Although nicotine and alcohol interact-not all the changes induced by the combined intake of both are additive-some are opposing. These opposing effects include those on locomotion, acetylcholine metabolism, nicotine binding, opiate peptides, glutamate transporters and endocannabinoid content among others. The two compounds lower the negative withdrawal symptoms of each other which may contribute to the increase in preference, but the mechanism by which preference increases-most likely consists of multiple components that are not clear at the present time. As the details of induced changes of nicotine and alcohol differ, it is likely that the mechanisms of increasing nicotine preference may not be identical to that of increasing alcohol preference. Stimulation of preference of yet other drugs may again be different -representing one aspect of drug specificity of reward mechanisms.

Varenicline and mecamylamine attenuate locomotor sensitization and cross-sensitization induced by nicotine and morphine in mice

The present study focused on the evaluation of behavioural sensitization and cross-sensitization induced by nicotine and morphine in mice. First, we revealed that after 9days of nicotine administration (0.175mg/kg, free base), every other day and following its 7-day withdrawal, challenge doses of nicotine (0.175mg/kg) and morphine (5mg/kg) induced locomotor sensitization in mice. When we examined the influence of varenicline, a partial alpha4beta2 nicotinic receptor agonist (0.5, 1 and 2mg/kg) and mecamylamine (0.5, 1 and 2mg/kg), a non-selective nicotinic receptor antagonist, we found that both agents attenuated the acquisition and expression of nicotine sensitization as well as locomotor cross-sensitization between nicotine and morphine. Our results indicate similar cholinergic mechanisms involved in the locomotor stimulant effects of nicotine and morphine in mice, and as such these data may suggest that nicotinic neurotransmission could be a potential target for developing pharmacotherapeutic strategies to treat and prevent nicotine and/or opioid addiction.

Endogenous opiates and behavior: 2008

This paper is the 31st consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2008 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

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.