SR141716, a central cannabinoid (CB(1)) receptor antagonist, blocks the motivational and dopamine-releasing effects of nicotine in rats

The neuronal pathways mediating the behavioral and addictive properties of nicotine

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

Recognising nicotine: the neurobiological basis of nicotine discrimination

Drug discrimination methodology makes possible the objective, quantitative study of the perception of psychoactive drug effects in either human or animal subjects. Investigations of the nicotine discriminative stimulus complex have contributed to our present understanding of nicotine psychopharmacology by defining the origin of its effects at specific subtypes of nicotinic receptor and the role of diverse neurotransmitter systems as mediating and modulating mechanisms. The evidence strongly supports central sites as the origins of the nicotine stimulus, and these are likely to be located in the mesocorticolimbic dopaminergic neurons; the medial prefrontal cortex is primarily involved, with the Nucleus accumbens and ventral tegmental area of secondary importance, while another element of the complex stimulus may arise in the dorsal hippocampus. Additionally, it appears that interactions of nicotine with the dopamine, serotonin, cannabinoid and probably glutamate systems all contribute to the final perceived stimulus. The resemblance between the nicotine discriminative stimulus and those of the psychomotor stimulant drugs amphetamine and cocaine contributes to defining the nature of the addictive properties of nicotine. It is particularly interesting that acute and chronic exposure to caffeine produce quantitative and qualitative changes in the characteristics of the nicotine stimulus. Interactions of nicotine with caffeine and cannabinoids strengthen proposals that the use of one substance serves as a "gateway" in sequential shifts of the target substance for drug-seeking behaviour, with profound implications for the human use of the substances concerned. Drug discrimination is also an important standard technique used in assessments of the abuse liability of novel psychoactive compounds, with relevance to attempts to develop novel nicotinic agonists for use as cognitive enhancers.

Pharmacotherapy for tobacco dependence

Pharmacotherapy can provide effective treatment of tobacco dependence and withdrawal, and thereby facilitate efforts to achieve and sustain tobacco abstinence. Currently approved medications for smoking cessation are nicotine replacement medications (NRT), including nicotine patch, gum, lozenge, sublingual tablet, inhaler and nasal spray, the antidepressant bupropion, and the nicotinic partial agonist varenicline. This review discusses the pharmacological basis for the use of these medications, and the properties that might contribute to their efficacy, safety, and abuse liability. The review also discusses how pharmacological principles can be used to improve existing medications, as well as assist in the development of new medications.

Effects of nicotine in experimental animals and humans: an update on addictive properties

Tobacco use through cigarette smoking is the leading preventable cause of death in the developed world. Nicotine, a psychoactive component of tobacco, appears to play a major role in tobacco dependence, but the reinforcing effects of nicotine have often been difficult to demonstrate directly in controlled studies with laboratory animals or human subjects. Here we update our earlier review published in Psychopharmacology (Berl) in 2006 on findings obtained with various procedures developed to study dependence-related behavioral effects of nicotine in experimental animals and humans. Results obtained with drug self-administration, conditioned place preference, subjective reports of nicotine effects and nicotine discrimination indicate that nicotine can function as an effective reinforcer of drug-seeking and drug-taking behavior both in experimental animals and humans under appropriate conditions. Interruption of chronic nicotine exposure produces ratings of drug withdrawal and withdrawal symptoms that may contribute to relapse. Difficulties encountered in demonstrating reinforcing effects of nicotine under some conditions, relative to other drugs of abuse, may be due to weaker primary reinforcing effects of nicotine, to aversive effects produced by nicotine, or to a more critical contribution of environmental stimuli to the maintenance of drug-seeking and drug-taking behavior with nicotine than with other drugs of abuse. Several recent reports suggest that other chemical substances inhaled along with nicotine in tobacco smoke may play a role in sustaining smoking behavior. However, conflicting results have been obtained with mice and rats and these findings have not yet been validated in nonhuman primates or human subjects. Taken together, these findings suggest that nicotine acts as a typical drug of abuse in experimental animals and humans in appropriate situations.

Drug addiction

Many drugs of abuse, including cannabinoids, opioids, alcohol and nicotine, can alter the levels of endocannabinoids in the brain. Recent studies show that release of endocannabinoids in the ventral tegmental area can modulate the reward-related effects of dopamine and might therefore be an important neurobiological mechanism underlying drug addiction. There is strong evidence that the endocannabinoid system is involved in drug-seeking behavior (especially behavior that is reinforced by drug-related cues), as well as in the mechanisms that underlie relapse to drug use. The cannabinoid CB(1) antagonist/inverse agonist rimonabant has been shown to reduce the behavioral effects of stimuli associated with drugs of abuse, including nicotine, alcohol, cocaine, and marijuana. Thus, the endocannabinoid system represents a promising target for development of new treatments for drug addiction.

Endogenous fatty acid ethanolamides suppress nicotine-induced activation of mesolimbic dopamine neurons through nuclear receptors

Nicotine stimulates the activity of mesolimbic dopamine neurons, which is believed to mediate the rewarding and addictive properties of tobacco use. Accumulating evidence suggests that the endocannabinoid system might play a major role in neuronal mechanisms underlying the rewarding properties of drugs of abuse, including nicotine. Here, we investigated the modulation of nicotine effects by the endocannabinoid system on dopamine neurons in the ventral tegmental area with electrophysiological techniques in vivo and in vitro. We discovered that pharmacological inhibition of fatty acid amide hydrolase (FAAH), the enzyme that catabolizes fatty acid ethanolamides, among which the endocannabinoid anandamide (AEA) is the best known, suppressed nicotine-induced excitation of dopamine cells. Importantly, this effect was mimicked by the administration of the FAAH substrates oleoylethanolamide (OEA) and palmitoylethanolamide (PEA), but not methanandamide, the hydrolysis resistant analog of AEA. OEA and PEA are naturally occurring lipid signaling molecules structurally related to AEA, but devoid of affinity for cannabinoid receptors. They blocked the effects of nicotine by activation of the peroxisome proliferator-activated receptor-alpha (PPAR-alpha), a nuclear receptor transcription factor involved in several aspects of lipid metabolism and energy balance. Activation of PPAR-alpha triggered a nongenomic stimulation of tyrosine kinases, which might lead to phosphorylation and negative regulation of neuronal nicotinic acetylcholine receptors. These data indicate for the first time that the anorexic lipids OEA and PEA possess neuromodulatory properties as endogenous ligands of PPAR-alpha in the brain and provide a potential new target for the treatment of nicotine addiction.

Inhibition of anandamide hydrolysis by cyclohexyl carbamic acid 3'-carbamoyl-3-yl ester (URB597) reverses abuse-related behavioral and neurochemical effects of nicotine in rats

Emerging evidence suggests that the rewarding, abuse-related effects of nicotine are modulated by the endocannabinoid system of the brain. For example, pharmacological blockade or genetic deletion of cannabinoid CB(1) receptors can reduce or eliminate many abuse-related behavioral and neurochemical effects of nicotine. Furthermore, doses of Delta(9)-tetrahydrocannabinol and nicotine that are ineffective when given alone can induce conditioned place preference when given together. These previous studies have used systemically administered CB(1) receptor agonists and antagonists and gene deletion techniques, which affect cannabinoid CB(1) receptors throughout the brain. A more functionally selective way to alter endocannabinoid activity is to inhibit fatty acid amide hydrolase (FAAH), thereby magnifying and prolonging the effects of the endocannabinoid anandamide only when and where it is synthesized and released on demand. Here, we combined behavioral and neurochemical approaches to evaluate whether the FAAH inhibitor URB597 (cyclohexyl carbamic acid 3'-carbamoyl-3-yl ester) could alter the abuse-related effects of nicotine in rats. We found that URB597, at a dose (0.3 mg/kg) that had no behavioral effects by itself, prevented development of nicotine-induced conditioned place preference (CPP) and acquisition of nicotine self-administration. URB597 also reduced nicotine-induced reinstatement in both CPP and self-administration models of relapse. Furthermore, in vivo microdialysis showed that URB597 reduced nicotine-induced dopamine elevations in the nucleus accumbens shell, the terminal area of the brain's mesolimbic reward system. These findings suggest that FAAH inhibition can counteract the addictive properties of nicotine and that FAAH may serve as a new target for development of medications for treatment of tobacco dependence.

Blockade of THC-seeking behavior and relapse in monkeys by the cannabinoid CB(1)-receptor antagonist rimonabant

Accumulating evidence suggests the endocannabinoid system modulates environmental cues' ability to induce seeking of drugs, including nicotine and alcohol. However, little attention has been directed toward extending these advances to the growing problem of cannabis use disorders. Therefore, we studied intravenous self-administration of Delta(9)-tetrahydrocannabinol (THC), the main psychoactive constituent of marijuana, using a second-order schedule of drug seeking. Squirrel monkeys' lever responses produced only a brief cue light until the end of the session, when the final response delivered THC along with the cue. When a reinstatement procedure was used to model relapse following a period of abstinence, THC-seeking behavior was robustly reinstated by the cue or by pre-session administration of THC, other cannabinoid agonists, or morphine, but not cocaine. The cannabinoid antagonist rimonabant blocked cue-induced drug seeking, THC-induced drug seeking, and the direct reinforcing effects of THC. Thus, rimonabant and related medications might be effective as treatments for cannabinoid dependence.

The endocannabinoid system: a new molecular target for the treatment of tobacco addiction

Tobacco addiction is one of the leading preventable causes of mortality in the world and nicotine appears to be the main critical psychoactive component in establishing and maintaining tobacco dependence. Several lines of evidence suggest that the rewarding effects of nicotine, which underlie its abuse potential, can be modulated by manipulating the endocannabinoid system. For example, pharmacological blockade or genetic deletion of cannabinoid CB(1) receptors reduces or eliminates many behavioral and neurochemical effects of nicotine that are related to its addictive potential. This review will focus on the recently published literature about the role of the endocannabinoid system in nicotine addiction and on the endocannabinoid system as a novel molecular target for the discovery of medications for tobacco dependence.

Effects of baclofen on conditioned rewarding and discriminative stimulus effects of nicotine in rats

Neurochemical studies suggest that baclofen, an agonist at GABA(B) receptors, may be useful for treatment of nicotine dependence. However, its ability to selectively reduce nicotine's abuse-related behavioral effects remains in question. We assessed effects of baclofen doses ranging from 0.1 to 3mg/kg on nicotine-induced conditioned place preferences (CPPs), nicotine discrimination, locomotor activity and food-reinforced behavior in male Sprague-Dawley rats. The high dose of baclofen (3mg/kg) totally eliminated food-reinforced responding and significantly decreased locomotor activity. Lower doses of baclofen did not have nicotine-like discriminative effects in rats trained to discriminate 0.4mg/kg nicotine from saline under a fixed-ratio 10 schedule of food delivery. Lower doses of baclofen also did not reduce discriminative stimulus effects of the training dose of nicotine and did not significantly shift the dose-response curve for nicotine discrimination. Rats treated with the high 3mg/kg dose of baclofen did not express nicotine-induced CPP. These experiments, along with previous reports that baclofen can reduce intravenous nicotine self-administration behavior, confirm the potential utility of baclofen as a tool for smoking cessation.

Contextual stimuli modulate extinction and reinstatement in rodents self-administering intravenous nicotine

RATIONALE

Discrete cues, such as drug-associated paraphernalia, play an important role in tobacco smoking and relapse, an effect that can be modelled in the nicotine-seeking behaviour of laboratory animals. However, the role of contextual stimuli (i.e. the drug taking environment) within nicotine dependence is less clear. The present study investigated the effects of contextual stimuli on nicotine detoxification and relapse.

MATERIALS AND METHODS

Male hooded Lister rats were trained to self-administer nicotine (0.03 mg/kg/infusion) in one of two distinct environmental contexts: transparent walls and rod floor or checkerboard walls and grid floor. Extinction of drug-seeking behaviour, either in the acquisition context or alternate context, was achieved by removing both nicotine infusions and response-contingent cues. The two contexts were then presented with or without nicotine priming and response-contingent cue presentation.

RESULTS

The initial rate of extinction was quicker in a novel environment compared to in the same context as training, although similar low levels of responding were eventually reached. Nicotine priming and re-presentation of cues resulted in significant reinstatement of nicotine-seeking behaviour, but there was a trend towards a reduction in this effect when conducted in a novel environment. In addition, re-presentation of the acquisition context after extinction in the alternate context produced a significant reinstatement of nicotine-seeking behaviour without the need for nicotine priming and re-presentation of cues.

CONCLUSIONS

Contextual stimuli are capable of modulating the extinction and reinstatement of nicotine-seeking behaviour, and exposure to environments previously associated with smoking may lead to an increased risk of relapse. Context is an additional factor that could be targeted when developing smoking cessation strategies. For example, the long-term success of cue exposure might be improved by conducting treatment in multiple settings.

Partial nicotinic acetylcholine (alpha4beta2) agonists as promising new medications for smoking cessation

OBJECTIVE

To review the pharmacology, clinical efficacy and safety of partial agonists of alpha4beta2 nicotinic acetylcholine receptor.

DATA SOURCES

Primary literature and review articles were obtained via a PUBMED search (1988-August 2006) using the key terms smoking cessation, partial agonist alpha4beta2 nicotinic acetylcholine receptor, varenicline, cytisine and SSR591813. Additional studies and abstracts were identified from the bibliographies of reviewed literature.

STUDY SELECTION AND DATA EXTRACTION

Studies and review articles related to varenicline, cytisine and the partial agonist alpha4beta2 nicotinic acetylcholine receptor were reviewed.

DATA SYNTHESIS

Smoking is widely recognized as a serious health problem. Smoking cessation has major health benefits. According to the US Public Health Services, all patients attempting to quit smoking should be encouraged to use one or more effective pharmacotherapy. Currently, along with nicotine replacement therapy, bupropion, nortriptyline and clonidine, are the mainstay of pharmacotherapy. More than (3/4) of patients receiving treatment for smoking cessation return to smoking within the first year. Nicotine, through stimulating alpha4beta2 nAChR, releases dopamine in the reward pathway. Partial agonist of alpha4beta2 nAChR elicits moderate and sustained release of dopamine, which is countered during the cessation attempts; it simultaneously blocks the effects of nicotine by binding with alpha4beta2 receptors during smoking. Recently, varenicline, a partial agonist at alpha4beta2 nAChR, has been approved by the FDA (Food and Drug Administration) for smoking cessation.

CONCLUSION

Partial agonist alpha4beta2 nAChR appears to be a promising target in smoking cessation. Varenicline of this group is approved for treatment of smoking cessation by the FDA in May 2006.

Ethanol self-administration is regulated by CB1 receptors in the nucleus accumbens and ventral tegmental area in alcohol-preferring AA rats

BACKGROUND

Endogenous cannabinoids and their receptors, CB1 receptors in particular, have been implicated in mediation of ethanol reinforcement. Previously, suppression of ethanol drinking by CB1 antagonists has been demonstrated in many experimental paradigms. However, the exact mechanism by which CB1 antagonists modulate ethanol drinking remains elusive. In the present study, we assessed the role of CB1 receptors within the key regions of the mesolimbic dopamine pathway, the nucleus accumbens (NAcc) and ventral tegmental area (VTA), in regulation of ethanol self-administration.

METHODS

Adult male alcohol-prefer AA rats were trained to self-administer either 10% (w/v) ethanol or 0.1% (w/v) saccharin under an FR1 schedule during daily 30-minute sessions. Following stable baseline responding, rats were tested after systemic administration of the CB1 antagonist SR141716A (0 to 10 mg/kg) and the agonist WIN55,212-2 (0 to 2 mg/kg). Separate groups of rats were implanted with bilateral cannulas aimed at the NAcc or VTA, and tested after microinjections of SR141716A (0 to 3 microg) and WIN55,212-2 (0 to 5 microg) into the NAcc or VTA. The highest intracerebral doses were tested also in rats responding for a 0.1% saccharin solution.

RESULTS

SR141617A dose-dependently suppressed ethanol responding after systemic administration. Microinjections of SR141617A both into NAcc and VTA attenuated ethanol responding. In addition, intra-NAcc injections of SR141617A suppressed saccharin intake. Although low doses of systemically given WIN55,212-2 increased ethanol responding, no effects were seen after WIN55,212-2 microinjections into NAcc or VTA.

CONCLUSIONS

Bidirectional changes in ethanol self-administration by the systematically administered CB1 agonist and antagonist show that ethanol reinforcement is controlled by CB1 receptors in alcohol-preferring AA rats. Replication of the suppressive effects by CB1 antagonism in the NAcc and VTA suggests that endocannabinoids and their receptors mediate ethanol reinforcement through interaction with the mesolimbic dopamine pathway.

[Neuropsychopharmacology of delta-9-tetrahydrocannabinol]

Today, the main route of introduction of tetrahydrocannabinol (THC), the main active substance of cannabis, into the human body is via the lungs, from smokes produced by combustion of a haschich-tobacco mixture. The use of a water pipe (nargileh-like) intensifies its fast supply to the body. THC reaches the brain easily where it stimulates CB1 receptors; their ubiquity underlies a wide variety of effects. THC disappears from extracellular spaces by dissolving in lipid rich membranes, and not by excretion from the body. This is followed by a slow release, leading to long lasting effects originating from brain areas containing a large proportion of spare receptors ("reserve receptors"). Far from mimicking the effects of endocannabinoids, THC caricatures and disturbs them. It induces both psychical and physical dependencies, but the perception of withdrawal is weak on account of its very slow elimination. THC disturbs cognition. Acutely, it develops anxiolytic- and antidepressant-like effects, which causes a lot of users to abuse THC, thus leading to a tolerance (desensitization of CB1 receptors) making anxiety and depression to reappear more intensely than originally. THC has close relationships with schizophrenia. It incites to tobacco, alcohol and heroine abuses.

Novel pharmacological approaches for treating tobacco dependence and withdrawal: current status

Increasing the diversity and availability of medications for the treatment of tobacco dependence and/or withdrawal, to aid in the achievement of smoking cessation, is crucial to meet the diverse needs of tobacco users. Despite a general awareness that smoking is harmful and widespread interest in smoking cessation, nearly 50 million adults in the US and 1.3 billion worldwide continue to smoke. Nicotine replacement therapies are effective in the treatment of tobacco dependence and withdrawal, but do not meet the needs of all tobacco users. Improvement of tobacco dependence and/or withdrawal treatments is likely to rely on novel pharmacological approaches that include new chemical entities and new formulations of current drugs. In addition, new indications for treating tobacco dependence and withdrawal show promise for reducing tobacco use and associated disease. This article focuses on a range of novel pharmacological approaches for the treatment of tobacco dependence and/or withdrawal, including oral and pulmonary nicotine delivery and the following non-nicotinic medications: antidepressants, an alpha4beta2 nicotine partial agonist, an alpha2-noradrenergic agonist, cytochrome P450 (CYP) 2A6 inhibitors, opioid antagonists and GABAergic medications. In addition to existing medications, this article addresses novel medications in the clinical development stage and those that have been evaluated previously. Novel medications in the clinical development stage include at least three nicotine vaccines and the cannabinoid receptor acting drug rimonabant. Medications evaluated previously include lobeline, mecamylamine and an anticholinergic drug regimen comprising atropine, scopolamine and chlorpromazine. Having not been approved by major drug regulatory authorities for the treatment of tobacco dependence and/or withdrawal, these medications have been evaluated in an experimental capacity.

The effect of cannabinoid CB(1) receptor antagonist rimonabant (SR-141716) on ethanol drinking in high-preferring rats

The present study investigated the effect of the cannabinoid CB(1) receptor antagonist, rimonabant (SR-141716) on ethanol intake in selectively bred alcohol-preferring Warsaw High-Preferring rats. Ethanol (10% vol/vol) and food were available in daily 4-h limited access period while water was available ad libitum. The administration (i.p.) of single 2.5 and 5.0-mg/kg doses of rimonabant preferentially reduced ethanol intake, whereas a 10 mg/kg dose of rimonabant similarly reduced both ethanol and food intake. Our result extends the suppressive effect of cannabinoid CB(1) receptor antagonist to the ethanol drinking behavior in Warsaw High-Preferring line of rats. The result also supports a growing body of literature indicating that the cannabinoid CB(1) receptor is involved in motivational and appetitive properties of ethanol.

Rimonabant blocks the expression but not the development of locomotor sensitization to nicotine in rats

RATIONALE

Cannabinoid, especially CB(1,) receptors have been implicated in the development and expression of a variety of behaviors produced by addictive drugs.

OBJECTIVES

The intent was to determine if coadministration of the selective CB(1) receptor antagonist, rimonabant (SR141716A), would block the development or expression of locomotor sensitization to repeated injections of nicotine.

MATERIALS AND METHODS

Male Long-Evans rats were injected with either 2 mg/kg rimonabant or its vehicle 30 min before an injection of 0.4 mg/kg nicotine or saline and immediately placed in activity chambers for 1 h on each of six sessions on alternating days. Before the two subsequent challenge sessions, all rats were injected with the vehicle and 0.4 mg/kg nicotine combination and then with the 2 mg/kg rimonabant and 0.4 mg/kg nicotine combination, respectively.

RESULTS

Repeated injections of nicotine produced a progressive increase in locomotion that was blocked by coadministration of rimonabant. However, the subsequent nicotine challenge increased locomotion in both nicotine-pretreated groups equally more than in the saline-pretreated groups. Coadministration of rimonabant along with nicotine on the second challenge decreased the locomotion of the nicotine-pretreated rats to equal that of the saline-pretreated rats. Rimonabant had no effect on the saline-pretreated rats.

CONCLUSION

These data suggest that rimonabant blocks the expression but not the development of locomotor sensitization to nicotine.

Tobacco addiction

Tobacco use is associated with 5 million deaths per year worldwide and is regarded as one of the leading causes of premature death. Comprehensive programmes for tobacco control can substantially reduce the frequency of tobacco use. An important component of a comprehensive programme is the provision of treatment for tobacco addiction. Treatment involves targeting several aspects of addiction including the underlying neurobiology and behavioural processes. Furthermore, building an infrastructure in health systems that encourages and helps with cessation, as well as expansion of the accessibility of treatments, is necessary. Although pharmacological and behavioural treatments are effective in improving cessation success, the rate of relapse to smoking remains high, emphasising the strong addictive nature of nicotine. The future of treatment resides in improvement in patient matching to treatment, combination or novel drugs, and viewing nicotine addiction as a chronic disorder that might need long-term treatment.

The role of CB1 receptors in psychostimulant addiction

Recent studies have implicated the endocannabinoid (eCB) system in the neuronal mechanisms underlying substance dependence. Here, we review results of studies using cannabinoid receptor subtype 1 (CB1) knockout mice as well as CB1 antagonists to elucidate the role of this neurotransmitter system in psychostimulant addiction. The overall picture is that CB1 receptors appear not to be involved in psychostimulant reward, nor in the development of dependence to such substances. In contrast, the eCB system appears to play a role in the persistence of psychostimulant addiction. In particular, CB1 receptors have been found to play a cardinal role in mediating reinstatement of previously extinguished drug-seeking behavior upon re-exposure to the drug or drug-associated cues. The anatomical loci as well as the neuronal mechanisms of the relapse-preventing effects of CB1 antagonists are still poorly understood, although interactions of the eCB system with afferent glutamatergic and possibly dopaminergic projections to the nucleus accumbens are most likely involved. In addition, CB1 receptors seem to modulate drug-related memories, in line with the hypothesized role of the eCB system in memory-related plasticity. Together, these findings suggest that modulators of the eCB system represent a promising novel type of therapy to treat drug addiction.

Blocking cannabinoid CB1 receptors for the treatment of nicotine dependence: insights from pre-clinical and clinical studies

Tobacco use is one of the leading preventable causes of death in developed countries. Since existing medications are only partially effective in treating tobacco smokers, there is a great need for improved medications for smoking cessation. It has been recently proposed that cannabinoid CB(1) receptor antagonists represent a new class of therapeutic agents for drug dependence, and notably, nicotine dependence. Here, we will review current evidence supporting the use of this class of drugs for smoking cessation treatment. Pre-clinical studies indicate that nicotine exposure produces changes in endocannabinoid content in the brain. In experimental animals, N-piperidinyl-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methylpyrazole-3-carboxamide (rimonabant, SR141716) and N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM251), two cannabinoid CB(1) receptor antagonists, block nicotine self-administration behavior, an effect that may be related to the blockade of the dopamine-releasing effects of nicotine in the brain. Rimonabant also seems efficacious in decreasing the influence of nicotine-associated stimuli over behavior, suggesting that it may act on two distinct neuronal pathways, those implicated in drug-taking behavior and those involved in relapse phenomena. The utility of rimonabant has been evaluated in several clinical trials. It seems that rimonabant is an efficacious treatment for smoking cessation, although its efficacy does not exceed that of nicotine-replacement therapy and its use may be limited by emotional side effects (nausea, anxiety and depression, mostly). Rimonabant also appears to decrease relapse rates in smokers. These findings indicate significant, but limited, utility of rimonabant for smoking cessation.

Effects of a Cannabinoid1 receptor antagonist and Serotonin2C receptor agonist alone and in combination on motivation for palatable food: a dose-addition analysis study in mice

The cannabinoid and serotonin systems modulate feeding behavior in humans and laboratory animals. The present study assessed whether a cannabinoid (CB)(1) receptor antagonist and a serotonin (5-HT)(2C) receptor agonist alone and in combination attenuate motivation for the liquid nutritional drink Ensure as measured by a progressive ratio (PR) schedule of reinforcement in male C57BL/6 mice. Pretreatment (15 min i.p.) with either the CB(1) receptor antagonist N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboximide hydrochloride (SR141716) (SR; Rimonabant or Acomplia) or the 5-HT(2C) receptor agonist m-chlorophenylpiperazine (mCPP) dose-dependently decreased the maximum ratio completed under the PR schedule (break point) in mice. ED(25) values for SR and mCPP to decrease break point were determined, and the relative potency of each drug alone was quantified. Fixed dose-ratio pairs of SR/mCPP based on their relative potency were then administered. Dose-addition analysis comparing the experimentally determined potency for SR/mCPP combinations with their predicted additive potency revealed that SR/mCPP combinations in 1:1 and 2:1 ratios based on relative potency produced significant synergistic attenuation of break point for Ensure. The ED(25) values for decreasing break point were consistently lower than ED(25) values for decreasing response rate, and synergistic effects of SR/mCPP combinations on break point were seen independent of synergistic effects on response rate. These results indicate that cannabinoid CB(1) and serotonin 5-HT(2C) receptors are involved in motivated feeding behavior in mice and that these compounds can synergistically modulate motivation for palatable food with the synergy dependent upon the ratio of SR/mCPP in the combination.

The endocannabinoid system in brain reward processes

Food, drugs and brain stimulation can serve as strong rewarding stimuli and are all believed to activate common brain circuits that evolved in mammals to favour fitness and survival. For decades, endogenous dopaminergic and opioid systems have been considered the most important systems in mediating brain reward processes. Recent evidence suggests that the endogenous cannabinoid (endocannabinoid) system also has an important role in signalling of rewarding events. First, CB(1) receptors are found in brain areas involved in reward processes, such as the dopaminergic mesolimbic system. Second, activation of CB(1) receptors by plant-derived, synthetic or endogenous CB(1) receptor agonists stimulates dopaminergic neurotransmission, produces rewarding effects and increases rewarding effects of abused drugs and food. Third, pharmacological or genetic blockade of CB(1) receptors prevents activation of dopaminergic neurotransmission by several addictive drugs and reduces rewarding effects of food and these drugs. Fourth, brain levels of the endocannabinoids anandamide and 2-arachidonoylglycerol are altered by activation of reward processes. However, the intrinsic activity of the endocannabinoid system does not appear to play a facilitatory role in brain stimulation reward and some evidence suggests it may even oppose it. The influence of the endocannabinoid system on brain reward processes may depend on the degree of activation of the different brain areas involved and might represent a mechanism for fine-tuning dopaminergic activity. Although involvement of the various components of the endocannabinoid system may differ depending on the type of rewarding event investigated, this system appears to play a major role in modulating reward processes.

Cannabinoids in health and disease

Cannabis sativa L. preparations have been used in medicine for millenia. However, concern over the dangers of abuse led to the banning of the medicinal use of marijuana in most countries in the 1930s. Only recently, marijuana and individual natural and synthetic cannabinoid receptor agonists and antagonists, as well as chemically related compounds, whose mechanism of action is still obscure, have come back to being considered of therapeutic value. However, their use is highly restricted. Despite the mild addiction to cannabis and the possible enhancement of addiction to other substances of abuse, when combined with cannabis, the therapeutic value of cannabinoids is too high to be put aside. Numerous diseases, such as anorexia, emesis, pain, inflammation, multiple sclerosis, neurodegenerative disorders (Parkinson's disease, Huntington's disease, Tourette's syndrome, Alzheimer's disease), epilepsy, glaucoma, osteoporosis, schizophrenia, cardiovascular disorders, cancer, obesity, and metabolic syndrome-related disorders, to name just a few, are being treated or have the potential to be treated by cannabinoid agonists/antagonists/cannabinoid-related compounds. In view of the very low toxicity and the generally benign side effects of this group of compounds, neglecting or denying their clinical potential is unacceptable--instead, we need to work on the development of more selective cannabinoid receptor agonists/antagonists and related compounds, as well as on novel drugs of this family with better selectivity, distribution patterns, and pharmacokinetics, and--in cases where it is impossible to separate the desired clinical action and the psychoactivity--just to monitor these side effects carefully.

Varenicline in the treatment of tobacco dependence

Varenicline, a partial agonist of alpha(4)beta(2) nicotinic acetylcholine receptors, is the most recently approved drug for smoking cessation. This paper reviews the outcomes of Phase 2 and Phase 3 clinical trials that assess the efficacy of varenicline in comparison to placebo and other smoking cessation pharmacotherapies, ie, sustained-release bupropion (bupropion SR) and nicotine transdermal patch. Varenicline has higher abstinence rates than placebo and the alternative active treatments at the end of standard regimen treatment periods. Significantly higher abstinence rates were also found with varenicline in comparison to both placebo and bupropion SR at the end of a 40-week non-treatment follow-up period. Varenicline typically tripled the abstinence rates compared with placebo. In addition, varenicline reduced craving and withdrawal symptoms as well as some of the positive experiences associated with smoking to a greater extent than placebo, bupropion SR, and nicotine replacement therapy (NRT). These findings are consistent with the proposed agonist/antagonist effects of varenicline. Preliminary studies assessing individual variables such as smoking dependency level and smoking reinforcement types provide justification to examine further the effects of varenicline according to these individual factors. Outcomes from such research could improve our understanding of varenicline's mechanism of action and could ultimately help clinicians to develop individualized smoking cessation programs. Also, given varenicline's ability to reduce the reward from smoking, it might be helpful to use it before cessation to motivate or prepare smokers for a quit attempt.

Update on pharmacologic options for smoking cessation treatment

Although the proportion of the adult population in the United States that smokes has decreased steadily, the rate of successful quit attempts is still low. Smokers develop nicotine dependence that resembles other addictions, and may require multiple attempts and long-term treatment to sustain abstinence. Currently available first-line agents for smoking cessation therapy include nicotine replacement therapy, which is available in several formulations, including transdermal patch, gum, nasal spray, inhaler, and lozenge; bupropion, an atypical antidepressant; and varenicline, a partial agonist of the alpha(4)beta(2) nicotinic acetylcholine receptor that was recently developed and approved specifically for smoking cessation therapy. Second-line agents are nortriptyline, a tricyclic antidepressant agent, and clonidine, an antihypertensive drug. With the exception of varenicline, which has been shown to offer significant improvement in abstinence rates over bupropion, all of the available treatments appear similarly effective. However, the adverse event profiles of nortriptyline and clonidine make them more appropriate for second-line therapy, when first-line treatments have failed or are not tolerated. Rimonabant, a cannabinoid-1 receptor antagonist that was being developed for smoking cessation, received a nonapprovable letter from the FDA in 2006 and there is no further information as to whether development for this indication is continuing for this agent. Nicotine vaccines are under investigation and offer promise, especially for relapse prevention. Ultimately, selection of pharmacologic agent should be based on the patient's comorbidities and preferences, as well as on the agent's adverse event profile.

Determination of endocannabinoid receptor antagonist SR141716 (rimonabant) in plasma by liquid chromatograph tandem mass spectrometry

SR141716 (rimonabant) is an endocannabinoid receptor antagonist. Endocannabinoids are a class of chemicals that affect neurotransmission via G-protein coupled CB1 (brain) and CB2 (peripheral tissue) receptors. Numerous animal studies have shown that SR141716 binds with the CB1 receptor in the brain, resulting in several biological consequences including reduced alcohol intake and reward as well as reduced food consumption. In this work, an analytical method based on liquid chromatography and electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS) has been developed and validated for the quantitative measurement of SR141716 in both human and rat plasma to support the investigation of this compound. A suitable internal standard (AM251) has been chosen and the experimental conditions have been optimized for the separation and detection of singly charged positive ions of SR141716 and the internal standard. A protein precipitation protocol has been developed for extraction of SR141716 and the internal standard from plasma samples. Quantitation was achieved using multiple-reaction-monitoring (MRM) mode for SR141716 (m/z 463-->m/z 363) and the internal standard (m/z 555-->m/z 455) and calibration curve over the concentration range of 5.00-1000 ng/ml was plotted using the peak-area ratio versus the concentration of SR141716 with a LOD and LLOQ of 1.09 and 3.62 ng/ml, respectively. The method developed has been used to analyze SR141716 in rat plasma samples from an animal study.

Cannabinoid CB1 antagonists and dopamine antagonists produce different effects on a task involving response allocation and effort-related choice in food-seeking behavior

RATIONALE

Cannabinoid CB1 antagonists/inverse agonists suppress food-motivated behaviors and are being evaluated as potential appetite suppressants. It has been suggested that the effects of CB1 antagonism on food motivation could be related to actions on mesolimbic dopamine (DA). If this were true, then the effects of interference with cannabinoid CB1 transmission should closely resemble the effects of interference with DA transmission.

OBJECTIVE

To directly compare the effects of DA antagonists with those of CB1 antagonists/inverse agonists, the present studies employed a concurrent lever-pressing/chow-intake procedure. With this task, interference with DA transmission shifts choice behavior such that lever pressing for a preferred food is decreased but chow intake is increased.

RESULTS

Rats treated with IP injections of the DA D1 antagonist SCH39166 (ecopipam; 0.05-0.2 mg/kg) or the D2 antagonist eticlopride (0.025-0.1 mg/kg) showed substantial decreases in lever pressing and concomitant increases in chow consumption. In contrast, IP administration of the CB1 neutral antagonist AM4113 (4.0-16.0 mg/kg) or the CB1 antagonist/inverse agonist AM251 (2.0-8.0 mg/kg) decreased operant responding for pellets, but there was no corresponding increase in chow intake.

CONCLUSIONS

These effects of CB1 antagonists/inverse agonists were similar to those produced by the appetite suppressant fenfluramine and by prefeeding. In contrast, low doses of DA antagonists leave primary food motivation intact, but shift behaviors toward food reinforcers that can be obtained with lower response costs. These results suggest that the effects of interference with CB1 transmission are readily distinguishable from those of reduced DA transmission.

Topiramate does not alter nicotine or cocaine discrimination in rats

The effects of topiramate, a potential treatment for drug dependence, were evaluated in two groups of rats trained to discriminate the administration of either 0.4 mg/kg nicotine or 10 mg/kg cocaine from that of saline, under a fixed-ratio 10 schedule of food delivery. Topiramate (1-60 mg/kg, intraperitoneal) did not produce any nicotine-like or cocaine-like discriminative effects by itself and did not produce any shift in the dose-response curves for nicotine or cocaine discrimination. Thus, the ability to discriminate the effects of nicotine or cocaine does not appear to be altered by topiramate administration. Furthermore, topiramate, given either alone or in combination with nicotine or cocaine, did not depress rates of responding. These experiments indicate that topiramate does not enhance or reduce the ability of rats to discriminate the effects of nicotine or cocaine.

Metabolic consequences of hyperglycemia and insulin resistance

Insulin is a pleiotropic hormone that exerts a multitude of effects on metabolism and various cellular processes in the body. The main metabolic actions of insulin are to stimulate glucose uptake in skeletal muscle and the heart and to suppress the production of glucose and very-low-density lipoprotein in the liver. Other metabolic effects of insulin include inhibition of glucose release from the liver, inhibition of the release of free fatty acids (FFAs) from adipose tissue, and stimulation of the process by which amino acids are incorporated into protein. Insulin resistance (IR) is a condition in which defects in the action of insulin are such that normal levels of insulin do not trigger the signal for glucose absorption. An excess of FFAs is implicated in the pathogenesis of IR. The effects of this condition can have profound pathophysiologic effects on various organs and tissues of the body. For example, IR is associated with impaired insulin signaling, impaired fibrinolysis, and inflammation. The clinical consequences include hyperglycemia-induced tissue damage, hypertension, dyslipidemia, metabolic syndrome, and cardiovascular disease. Pharmacotherapies that target IR include metformin and the thiazolidinediones. Endocannabinoid antagonists, agents that target obesity and associated cardiovascular and metabolic risk factors, are currently being developed.

Smoking cessation therapy with varenicline

Smoking cessation is the only available intervention proven to halt progression of chronic obstructive pulmonary disease (COPD). The authors discuss the current existing treatment modalities and the role of a newly approved agent, varenicline, in promotion of smoking cessation. Varenicline is a novel agent that is a centrally acting partial nicotinic acetylcholine receptor agonist. It has both agonistic and antagonistic properties that together are believed to account for reduction of craving and withdrawal as well as blocking the rewarding effects of smoking. Its targeted mechanism of action, better efficacy and tolerability makes varenicline a useful therapeutic option for smoking cessation. In this article, we discuss presently available options for smoking cessation and review the literature on efficacy of varenicline.

The cannabinoid antagonist AM251 attenuates nicotine self-administration and nicotine-seeking behaviour in rats

The cannabinoid receptor subtype (CB1) antagonist rimonabant (SR141716) has been shown to decrease nicotine self-administration and attenuate nicotine-evoked dopamine release in the nucleus accumbens; effects that support recent findings on its clinical efficacy as a smoking cessation aid. The present experiments aim to advance our understanding on the role of CB1 receptors in rodent models of nicotine dependence. AM251, a selective antagonist at CB1 receptors dose-dependently (1, 3 and 10mg/kg IP) suppressed intravenous nicotine (0.03mg/kg per infusion) self-administration in rats during three successive days of pre-treatment. This reduction was short lasting since behaviour was reinstated by suspending AM251 pre-treatment. This was relatively specific to nicotine self-administration since the profile of these reductions produced by AM251 was significantly different from the responses maintained by food pellets. In a model of nicotine-seeking behaviour, rats that had been extinguished by removal of nicotine and associated cues, and presented with a priming dose of nicotine (0.2mg/kg SC) with the cues, showed robustly reinstated responses to nicotine-seeking behaviour. Acute pre-treatment with AM251 (1-10mg/kg IP) dose-dependently attenuated the reinstatement effects produced by nicotine and the contingently presented cues. These preclinical findings support the use of rimonabant as a smoking cessation aid and highlight the CB1 receptor as a viable target to control intake of nicotine and prevent relapse.

Role of endocannabinoids in regulating drug dependence

This review will discuss the latest knowledge of how the endocannabinoid system might be involved in treating addiction to the most common illicit drugs. Experimental models are providing increasing evidence for the pharmacological management of endocannabinoid signaling not only to block the direct reinforcing effects of cannabis, opioids, nicotine and ethanol, but also for preventing relapse to the various drugs of abuse, including opioids, cocaine, nicotine, alcohol and metamphetamine. Preclinical and clinical studies suggest that the endocannabinoid system can be manipulated by the CB1 receptor antagonist SR141716A, that might constitute a new generation of compounds for treating addiction across different classes of abused drugs.

Single and multiple doses of rimonabant antagonize acute effects of smoked cannabis in male cannabis users

RATIONALE

A single 90-mg dose of the cannabinoid CB1 receptor antagonist rimonabant attenuates effects of smoked cannabis in humans.

OBJECTIVES

The objective of this study is to evaluate whether repeated daily 40-mg doses of rimonabant can attenuate effects of smoked cannabis to the same extent as a single higher (90 mg) dose.

MATERIALS AND METHODS

Forty-two male volunteers received one of three oral drug regimens in a randomized, double blind, parallel group design: (1) 40 mg rimonabant daily for 15 days, (2) placebo for 14 days, then 90 mg rimonabant on day 15, or (3) placebo for 15 days. All participants smoked an active or placebo cannabis cigarette 2 h after medication on days 8 and 15. Subjective effects were measured with visual analog scales and the marijuana-scale of the Addiction Research Center Inventory.

RESULTS

Cannabis-induced tachycardia was significantly lower for the 40-mg group on day 8 and for the 40 and 90 mg rimonabant groups on day 15 as compared to placebo. The 40-mg dose significantly decreased peak subjective effects on day 8. Neither the 90-mg nor 40-mg doses significantly decreased peak subjective effects on day 15. Rimonabant treatment did not significantly affect Delta(9)-tetrahydrocannabinnol pharmacokinetics.

CONCLUSIONS

Repeated lower daily rimonabant doses (40 mg) attenuated the acute physiological effects of smoked cannabis to a similar degree as a single 90-mg dose; repeated 40-mg doses attenuated subjective effects after 8 but not 15 days.

Endocannabinoid system involvement in brain reward processes related to drug abuse

Cannabis is the most commonly abused illegal drug in the world and its main psychoactive ingredient, delta-9-tetrahydrocannabinol (THC), produces rewarding effects in humans and non-human primates. Over the last several decades, an endogenous system comprised of cannabinoid receptors, endogenous ligands for these receptors and enzymes responsible for the synthesis and degradation of these endogenous cannabinoid ligands has been discovered and partly characterized. Experimental findings strongly suggest a major involvement of the endocannabinoid system in general brain reward functions and drug abuse. First, natural and synthetic cannabinoids and endocannabinoids can produce rewarding effects in humans and laboratory animals. Second, activation or blockade of the endogenous cannabinoid system has been shown to modulate the rewarding effects of non-cannabinoid psychoactive drugs. Third, most abused drugs alter brain levels of endocannabinoids in the brain. In addition to reward functions, the endocannabinoid cannabinoid system appears to be involved in the ability of drugs and drug-related cues to reinstate drug-seeking behavior in animal models of relapse. Altogether, evidence points to the endocannadinoid system as a promising target for the development of medications for the treatment of drug abuse.

The peripheral cannabinoid receptor knockout mice: an update

This review gives an overview of the CB2 receptor (CB2R) knockout (CB2R-/-) mice phenotype and the work that has been carried out using this mutant mouse. Using the CB2R-/- mice, investigators have discovered the involvement of CB2R on immune cell function and development, infection, embryonic development, bone loss, liver disorders, pain, autoimmune inflammation, allergic dermatitis, atherosclerosis, apoptosis and chemotaxis. Using the CB2R-/- mice, investigators have also found that this receptor is not involved in cannabinoid-induced hypotension. In addition, the CB2R-/- mice have been used to determine specific tissue CB2R expression. The specificity of synthetic cannabinoid agonists, antagonists and anti-CB2R antibodies has been screened using tissues from CB2R-/- mice. Thus, the use of this mouse model has greatly helped reveal the diverse events involving the CB2R, and has aided in drug and antibody screening.

The anandamide transport inhibitor AM404 reduces ethanol self-administration

The endocannabinoid system mediates in the pharmacological actions of ethanol and genetic studies link endocannabinoid signaling to alcoholism. Drugs activating cannabinoid CB1 receptors have been found to promote alcohol consumption but their effects on self-administration of alcohol are less clear because of the interference with motor performance. To avoid this problem, a novel pharmacological approach to the study of the contribution of the cannabinoid system in alcoholism may be to use drugs that locally amplify the effects of alcohol on endogenous cannabinoids. In the present study we addressed this model by studying the effects of the anandamide transport inhibitor N-(4-hydroxyphenyl) arachidonoyl-ethanolamide (AM404) on both ethanol self-administration and reinstatement of alcohol-seeking behavior in rats. The results show that AM404 significantly reduced ethanol self-administration in a dose-dependent manner but failed to modify reinstatement for lever pressing induced by the stimulus associated with alcohol. This effect was not due to a motor depressant effect and was not related to a decrease in general motivational state, as it was not effective in other reward paradigms such as lever pressing for a saccharin solution. The mechanism of action of AM404 does not involve cannabinoid CB1 receptors as the CB1-selective antagonist SR141716A did not block the reduction of ethanol self-administration induced by the anandamide uptake blocker. Moreover, 3-(1,1-dimethylheptyl)-(-)-11-hydroxy-delta 8-tetrahydrocannabinol (HU-210), a classical cannabinoid receptor agonist, did not affect ethanol self-administration. The effects of AM404 are not mediated by either vanilloid VR1 receptors or cannabinoid CB2 receptors because it is not antagonized by either the VR1 receptor antagonist capsazepine or the CB2 antagonist AM630. These results indicate that AM404 may be considered as an innovative approach to reduce alcohol consumption.

Cortico-limbic circuitry for conditioned nicotine-seeking behavior in rats involves endocannabinoid signaling

RATIONALE

The endocannabinoid system plays an important role in conditioned drug seeking, but the neuronal mechanisms involved in this behavior are unclear.

OBJECTIVES

Here, we evaluate the role of endogenous cannabinoids in the cortico-limbic circuitry in cue-induced nicotine-seeking behavior in rats.

METHODS

Animals were first trained to self-administer nicotine (0.03 mg/kg/injection, IV) under conditions in which responding was reinforced jointly by response-contingent nicotine injections and audiovisual stimuli. During subsequent sessions, nicotine was withdrawn and responding was reinforced by contingent presentation of the stimuli only. One month after nicotine removal, the cannabinoid CB1 receptor antagonist, rimonabant, was injected bilaterally into the shell of the nucleus accumbens (ShNAcc, 0.3, 3, or 30 ng/0.5 microl), the basolateral amygdala (BLA, 30 ng/0.5 microl), or the prelimbic cortex (PLCx, 30 ng/0.5 microl).

RESULTS

Rimonabant injected into the ShNAcc dose-dependently reduced nicotine-seeking behavior without modifying spontaneous locomotor activity. Similar results were obtained when the drug (30 ng) was injected into the BLA or the PLCx. The anatomical specificity was confirmed in a control experiment using [(3)H]rimonabant. Fifteen minutes after drug injection, when the behavioral effects of rimonabant were already achieved, radioactivity was detected at the site of injection and had not diffused to adjacent regions.

CONCLUSIONS

These findings demonstrate that increased endocannabinoid transmission critically triggers conditioned nicotine-seeking behavior in key cortico-limbic regions.

Endocannabinoid regulation of relapse mechanisms

Addiction involves a complex neuropharmacologic behavioural cycle, in which positive reinforcement exerted by the drug and the negative state of withdrawal drive the user to extremes to obtain the drug. Comprehensive studies have established that relapse is the most common outcome of recovery programs treating addictive behaviours. Several types of anticraving medication are available nowadays, such as naltrexone for the treatment of alcoholism, bupropion for nicotine, methadone or buprenorphine for heroin. This review focuses on recent behavioural data providing a rationale for an endocannabinoid mechanism underlying reinstatement of compulsive drug seeking. Studies supporting the contention that reinstatement of extinguished drug self-administration behaviour may be generated by cannabinoid CB1 receptor agonists and attenuated, if not blocked, by CB1 receptor antagonists, are here reviewed. In support to these findings, conditioned place preference studies substantiate the involvement of the endocannabinoid system in recidivism mechanisms by demonstrating that motivation to relapse can be triggered by CB1 receptor activation while blockade of such receptors may prevent reinstatement of place conditioning induced by either drug primings or drug-associated cues. Finally, biochemical studies evaluating changes in endocannabinoid levels, CB1 receptor density and CB1 mRNA expression during re-exposure to drug following extinction are also examined. Taken together, the evidence available has important implications in the understanding and treatment of relapsing episodes in patients undergoing detoxification.

Translational research in medication development for nicotine dependence

A major obstacle to the development of medications for nicotine dependence is the lack of animal and human laboratory models with sufficient predictive clinical validity to support the translation of knowledge from laboratory studies to clinical research. This Review describes the animal and human laboratory paradigms commonly used to investigate the pathophysiology of nicotine dependence, and proposes how their predictive validity might be determined and improved, thereby enhancing the development of new medications.

Emerging pharmacotherapies for smoking cessation

PURPOSE

The neurobiology of tobacco dependence and the efficacy and safety of emerging pharmacotherapies for smoking cessation are reviewed.

SUMMARY

Dopamine is pivotal to the major addictive properties of nicotine. The neurotransmitters that influence dopamine activity include gamma-aminobutyric acid, acetylcholine, glutamate, norepinephrine, and serotonin. Opioids and endocannabinoids can also affect dopamine activity. Research on pharmacologic treatments for nicotine dependence has targeted the modulation of these neurotransmitter systems. Current pharmacotherapies for smoking cessation include bupropion and varenicline, both of which target the neurotransmitters involved in nicotine addiction. Several new therapies are emerging as possible treatment options for smoking cessation. Rimonabant, a selective cannabinoid antagonist, blocks dopamine release in the nucleus accumbens, a primary reward center for the brain. Studies have found that rimonabant may not only be effective as a smoking-cessation aid but may also assist in the maintenance of nicotine abstinence. Rimonabant has also demonstrated a weight-loss benefit, which may be attractive to smokers concerned with weight gain associated with smoking cessation. Three nicotine vaccines are currently in development, each acting to sequester nicotine from the bloodstream, thereby preventing its penetration of the central nervous system. Ongoing studies will evaluate their use as established therapies for smoking cessation. New nicotine-replacement formulations are also being developed.

CONCLUSION

There are several promising products in development targeting the mechanisms of tobacco dependence. As failure rates are high and relapse is common, these emerging therapies would offer more therapeutic options for smoking cessation and solutions to the problem of relapse.

Blocking the Cannabinoid Receptors: Drug Candidates and Therapeutic Promises

The CB1 and CB2 cannabinoid receptors have been described as two prime sites of action for endocannabinoids. Both the localization and pharmacology of these two G-protein-coupled receptors are well-described, and numerous selective ligands have been characterized. The physiological effects of Cannabis sativa (cannabis) and a throughout study of the endocannabinoid system allowed for the identification of several pathophysiological conditions--including obesity, dyslipidemia, addictions, inflammation, and allergies--in which blocking the cannabinoid receptors might be beneficial. Many CB1 receptor antagonists are now in clinical trials, and the results of several studies involving the CB1 antagonist lead compound rimonabant (SR141716A) are now available. This review describes the pharmacological tools that are currently available and the animal studies supporting the therapeutic use of cannabinoid receptor antagonists and inverse agonists. The data available from the clinical trials are also discussed.

The fatty acid amide hydrolase C385A (P129T) missense variant in cannabis users: studies of drug use and dependence in Caucasians

A genetic variation in fatty acid amide hydrolase (FAAH), C385A (P129T), has been previously associated with risk for problem street drug use. FAAH is a mammalian enzyme that inactivates neuromodulatory-signaling lipids including the endogenous cannabinoid 1 receptor agonist anandamide. We investigated in adult Caucasians (N = 749) whether this FAAH variant altered the risk for trying, regular use of or dependence on cannabis, alcohol or nicotine, traditional "gateway" drugs. Consistent with our knowledge that the A/A genotype results in reduced FAAH expression and activity in humans, subjects with the A/A genotype were less likely to be THC dependent than subjects with either a C/C or C/A genotype (11% vs. 26%, P < 0.05). No association was observed between the A/A genotype and risk for alcohol or tobacco regular use, or DSM IV dependence. Controlling for regular use of nicotine and sedatives, both identified as confounders, those with the A/A genotype were at significantly reduced risk for being THC dependent (OR 0.25, 95% CI: 0.07-0.88) as compared with those with the C/A or C/C genotype, supporting a link between alterations in the endocannabinoid system and THC dependence. Unexpectedly, we found an increased risk for regular use of sedatives among the A/A genotype group. The relationship between the FAAH A/A genotype and risk for drug dependence in this study was drug class specific, suggesting it is not part of a more general drug abuse effect. These results, particularly the observation of altered risk for sedative drug use, should be investigated further in multiple ethnic populations.

The cannabinoid CB1 receptor antagonist, rimonabant, as a promising pharmacotherapy for alcohol dependence: preclinical evidence

Several lines of preclinical evidence indicate the ability of the prototypic cannabinoid CB(1) receptor antagonist, rimonabant, to suppress various alcohol-related behaviors, including alcohol drinking and seeking behavior and alcohol self-administration in rats and mice. Together, these data-synthetically reviewed in the present paper-suggest (a) the involvement of the cannabinoid CB(1) receptor in the neural substrate controlling alcohol intake, alcohol reinforcement, and the motivational properties of alcohol and (b) that rimonabant may constitute a new and potentially effective medication for the treatment of alcohol dependence.

Rimonabant: just an antiobesity drug? Current evidence on its pleiotropic effects

The advent of the highly selective cannabinoid receptor (CB1) antagonist, rimonabant (SR141716; Acomplia) can revolutionize the ability of the clinicians to manage obesity. Large-scale clinical trials have demonstrated that rimonabant therapy can reduce obesity. Although, the precise mechanisms of action of rimonabant have to be further dissected, it is emerging, from both preclinical and clinical research, that not only is rimonabant an antiobesity drug, but also its pleiotropic functions affect a broad range of diseases, from obesity-related comorbidities to drug dependence and cancer. Here we review recent data from the literature and discuss the full pharmacological potential of this drug.

Pharmacological approaches to smoking cessation

Smoking, the most prominent nongenetic factor contributing to mortality, remains the major public health problem throughout the world. There are nearly 1.1 billion users of nicotine and tobacco products worldwide while approximately one third to half of them will die from smoking-related disease. The habit of smoking is mainly propelled by nicotine, a strongly addictive substance, to which the vast majority of smokers fall victim. Except for the general and specific support and counseling strategies there are now effective treatments for nicotine addiction. Two types of pharmacological therapies have been approved and are now licensed for smoking cessation. The first therapy consists of nicotine replacement, substituting the nicotine from cigarettes with safer nicotine formulations. The second therapy is bupropion, an antidepressant of the aminoketone class, which has been demonstrated to be effective in smoking cessation. However, although some cigarette smokers are able to quit, many are not, and standard medications to assist smoking cessation are ineffective. Several agents used for other indications (e.g. neurological diseases, depression, alcoholism) might be used to treat this subgroup. In conclusion, new more effective drugs are needed in order to fight the panepidemic of smoking globally.