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

Role of cannabinoidergic mechanisms in ethanol self-administration and ethanol seeking in rat adult offspring following perinatal exposure to Delta9-tetrahydrocannabinol

The present study evaluated the consequences of perinatal Delta(9)-tetrahydrocannabinol (Delta(9)-THC) treatment (5 mg/kg/day by gavage), either alone or combined with ethanol (3% v/v as the only fluid available), on ethanol self-administration and alcohol-seeking behavior in rat adult offspring. Furthermore, the effect of the selective cannabinoid CB(1) receptor antagonist, SR-141716A, on ethanol self-administration and on reinstatement of ethanol-seeking behavior induced either by stress or conditioned drug-paired cues was evaluated in adult offspring of rats exposed to the same perinatal treatment. Lastly, microarray experiments were conducted to evaluate if perinatal treatment with Delta(9)-tetrahydrocannabinol, ethanol or their combination causes long-term changes in brain gene expression profile in rats. The results of microarray data analysis showed that 139, 112 and 170 genes were differentially expressed in the EtOH, Delta(9)-THC, or EtOH+Delta(9)-THC group, respectively. No differences in alcohol self-administration and alcohol seeking were observed between rat groups. Intraperitoneal (IP) administration of SR-141716A (0.3-3.0 mg/kg) significantly reduced lever pressing for ethanol and blocked conditioned reinstatement of alcohol seeking. At the same doses SR-141716A failed to block foot-shock stress-induced reinstatement of alcohol seeking. The results reveal that perinatal exposure to Delta(9)-THC ethanol or their combination results in evident changes in gene expression patterns. However, these treatments do not significantly affect vulnerability to ethanol abuse in adult offspring. On the other hand, the results obtained with SR-141716A emphasize that endocannabinoid mechanisms play a major role in ethanol self-administration, as well as in the reinstatement of ethanol-seeking behavior induced by conditioned cues, supporting the idea that cannabinoid CB(1) receptor antagonists may represent interesting agents for the pharmacotherapy of alcoholism.

Pharmacotherapy of dual substance abuse and dependence

The US FDA has approved a limited number of treatments for alcohol, nicotine and opioid dependence; however, no treatments for other abused drugs such as marijuana, cocaine or methamphetamine are approved. This review focuses on research into drug pharmacotherapies, particularly single-drug therapies, for substance abuse and dependence contributing to the most important dual substance use disorders (SUDs). Given the implications of poly-substance abuse, it is essential that clinicians and researchers be aware of potential pharmacotherapies for the treatment of dual SUDs.A substantial number of patients abuse more than one drug concurrently, complicating the treatment of SUD and leaving clinicians with few FDA-approved drug options for their patients. In this era of evidence-based medicine, such patients are typically treated with therapeutically proven medications, but in ways that are outside the scope of a drug's original indication by the FDA. Such 'off-label' prescribing has become an important therapeutic strategy for practitioners seeking treatments for other diseases in subpopulations such as paediatrics and gerontology or for medical conditions such as oncology or mental illness. Similarly, the information that most clinicians use to make their decisions for treating patients abusing multiple drugs stems from trials treating a single SUD, anecdotal experiences from their own practice or that of their colleagues, or single-case studies reported in the literature. The existing evidence suggests there are few treatments for SUDs that confer significant reductions in substance use across a broad patient population. Moreover, even fewer clinical efficacy trials have been conducted that provide evidence of therapeutic benefit for these drugs. Recognising the difficulty in making the proper drug choice for facilitating maximum treatment success, this review highlights the single drugs or drug combinations that show some potential for treating dual SUDs. This review finds strongest support for the use of disulfiram for treatment of alcohol and cocaine dependence (with or without concomitant methadone maintenance), baclofen for alcohol and cocaine dependence (but not opioid-dependent cocaine users), tiagabine for cocaine dependence in methadone-maintained patients, and topiramate for alcohol, nicotine and cocaine dependence. While ondansetron and olanzapine show some efficacy in treating alcohol and cocaine dependence, more research is needed to better delineate the subpopulation in which these drugs may provide their maximum effect.

The lack of CB1 receptors prevents neuroadapatations of both NMDA and GABA(A) receptors after chronic ethanol exposure

As the contribution of cannabinoid (CB1) receptors in the neuroadaptations following chronic alcohol exposure is unknown, we investigated the neuroadaptations induced by chronic alcohol exposure on both NMDA and GABA(A) receptors in CB1-/- mice. Our results show that basal levels of hippocampal [(3)H]MK-801 ((1)-5-methyl-10,11-dihydro-5Hdibenzo[a,d]cyclohepten-5,10-imine) binding sites were decreased in CB1-/- mice and that these mice were also less sensitive to the locomotor effects of MK-801. Basal level of both hippocampal and cerebellar [(3)H]muscimol binding was lower and sensitivity to the hypothermic effects of diazepam and pentobarbital was increased in CB1-/- mice. GABA(A)alpha1, beta2, and gamma2 and NMDA receptor (NR) 1 and 2B subunit mRNA levels were altered in striatum of CB1-/- mice. Our results also showed that [(3)H]MK-801 binding sites were increased in cerebral cortex and hippocampus after chronic ethanol ingestion only in wild-type mice. Chronic ethanol ingestion did not modify the sensitivity to the locomotor effects of MK-801 in both genotypes. Similarly, chronic ethanol ingestion reduced the number of [(3)H]muscimol binding sites in cerebral cortex, but not in cerebellum, only in CB1+/+ mice. We conclude that lifelong deletion of CB1 receptors impairs neuroadaptations of both NMDA and GABA(A) receptors after chronic ethanol exposure and that the endocannabinoid/CB1 receptor system is involved in alcohol dependence.

The cannabinoid antagonist SR 141716A (Rimonabant) reduces the increase of extra-cellular dopamine release in the rat nucleus accumbens induced by a novel high palatable food

The assumption of a novel high palatable food (a candied cherry) occurs concomitantly with an increase in the concentration of extra-cellular dopamine and its main metabolite 3,4-dihydroxy-phenylacetic acid (DOPAC) by about 45% in the dialysate obtained by intracerebral microdialysis from the shell of the nucleus accumbens of male rats. Such increase was reversed by SR 141716A (Rimonabant), a selective cannabinoid CB1 receptor antagonist (0.3 mg/kg i.p. and 1 mg/kg i.p.), which also reduces the assumption of the high palatable food, when given 15 min before exposure to the candied cherry. SR 141716A effects on extracellular dopamine and DOPAC were prevented by WIN 55,212-2 (0.3 mg/kg i.p.) or HU 210 (0.1 mg/kg i.p.) given 15 min before SR 141716A. The present results show for the first time that SR 141716A reduces the increase in extra-cellular dopamine induced by a novel high palatable food in the nucleus accumbens. This confirms that cannabinoid CB1 receptors play a key role in food intake and/or appetite and suggests that the mesolimbic dopaminergic system is involved at least in part, in the effects of cannabinoid receptor agonists and antagonists on food intake and/or appetite.

Integrated physiology and pathophysiology of CB1-mediated effects of the endocannabinoid system

The discovery of the endocannabinoid system (ECS) has raised a large interest in the scientific community providing us with a strikingly long list of apparently independent multi organ effects. As a result, in most reviews on this issue the main function of the ECS is considered as modulatory. Unfortunately, this vision does not add much to our understanding of the specific biological function of the ECS. Thus, modulatory is what in general all biological systems are or should be. In this review we will show that the apparent inconsistent puzzle of the very different tissue specific effects of endocannabinoids (ECs) can be reconstructed in one unitary picture. This picture clearly shows that all the different CB1-mediated effects of ECs sub-serve one major physiological function: to facilitate and increase energy storage. We will also analyze the implications of this unitary vision of the ECS in different contexts. First, in the context of the systems that regulate energy balance, introducing a new systematization based on two homeostatic systems: an endostatic and an exostatic system. Second, in the context of evolution, showing how the function of the ECS has shifted from essential to survival to almost pathological in current times. Finally, in a pathophysiological context, introducing the new concept of "proactive evolution diseases", which can explain the current obesity epidemic and the role the ECS plays in it.

Medial forebrain bundle stimulation evokes endocannabinoid-mediated modulation of ventral tegmental area dopamine neuron firing in vivo

RATIONALE

Endocannabinoid-mediated forms of transient synaptic depression have been described in several brain structures, including the dopaminergic ventral tegmental area (VTA). However, their functional and/or behavioural correlates are yet to be determined.

OBJECTIVES

The present study was designed to investigate whether back-propagating action potentials in dopamine (DA) neurons, evoked by the stimulation of the medial forebrain bundle (MFB), could trigger endocannabinoid-mediated forms of synaptic modulation. The MFB contains axons ascending from DA neurons to the nucleus accumbens and other forebrain structures, and its stimulation is rewarding because it elicits intra-cranial self-stimulation.

MATERIALS AND METHODS

Single cell extracellular recordings were carried out from anti-dromically identified VTA DA neurons in chloral hydrate anesthetized rats.

RESULTS

DA neurons responded to MFB stimulation (1 s, 20-80 Hz) with a frequency-dependent increase in spontaneous firing rate, which was enhanced by the cannabinoid type-1 receptor antagonist SR141716A (1 mg/kg) and depressed by the agonist WIN55212-2 (0.125 mg/kg). Increasing brain levels of the endocannabinoid anandamide by blocking its major hydrolysing enzyme, fatty-acid amide hydrolase, with URB597 (0.1 mg/kg) was ineffective, whereas blockade of the endocannabinoid membrane transporter with UCM707 (1 mg/kg) enhanced post-stimulus firing rate.

CONCLUSIONS

Our study indicates that stimulation of the MFB evokes an endocannabinoid-mediated short-term modulation of DA neuron activity. Thus, endocannabinoids might play an important role in the mechanisms underlying the rewarding properties of MFB stimulation.

The CB1 cannabinoid receptor antagonist rimonabant chronically prevents the nicotine-induced relapse to alcohol

Preclinical and clinical research shows that the cannabinoid brain receptor type 1 (CB(1)) modulates alcohol- and nicotine-related behaviors. Throughout the nicotine-induced relapse to alcohol, the rats were pre-treated for 10 days with the CB(1) cannabinoid receptor antagonist rimonabant (0, 0.03, 0.3 and 3.0 mg/kg i.p.). In this condition, a long-lasting nicotine-induced relapse to alcohol was observed, and this effect was reversed in a dose-dependent manner with rimonabant. Surprisingly, rats that were not exposed to nicotine developed tolerance to the effects of rimonabant from the sixth day. Also, 3.0 mg/kg of rimonabant reduced the responses for sucrose. Evaluation in the Elevated Plus-Maze after nicotine treatment did not reveal anxiogenic effects. Finally, at the conclusion of rimonabant treatment, a rapid reinstatement of alcohol consumption was detected. These results suggest that rimonabant can prevent the relapse to alcohol, even when an interaction with nicotine exists-the most frequent situation in human alcohol abuse.

Pharmakologische Rückfallprävention bei Alkohol- und Tabakabhängigkeit

A better understanding of the neurobiological substrates of alcohol and tobacco dependence has been accompanied by the increasing role of pharmacological relapse prevention. In alcohol dependence, substances which are able to block or ameliorate alcohol craving improve the maintenance of abstinence in combination with psychosocial treatment. In particular, the N-methyl-D-aspartate receptor antagonist acamprosate and the opiate receptor antagonist naltrexone have been shown to be effective in numerous trials. Administration starts immediately after detoxification and should last for 12 months. This treatment, however, is not a replacement for the psychosocial treatment of the alcohol dependence. Compared with placebo treatment, approximately twice as many patients remain abstinent under acamprosate 1 year after the end of treatment. The most widely studied and used pharmacotherapy for the treatment of tobacco dependence is nicotine containing medications. Nicotine replacement therapies (patch, gum, tablet, inhaler) have been shown to reduce tobacco craving and enhance abstinence. The only non-nicotine medication that has been approved in Germany is the antidepressant bupropion. Current studies identify particularly suitable subgroups of patients responding to more individualized pharmacological relapse prevention. A specific application of these substances will optimise the outcome for alcohol and tobacco dependent patients.

WIN-55,212-2 and SR-141716A alter nicotine-induced changes in locomotor activity, but do not alter nicotine-evoked [3H]dopamine release

Nicotine, the main psychoactive ingredient in tobacco, plays a key role in the development of cigarette smoking addiction. The endocannabinoid system has been demonstrated to have an important role in the motivational and reinforcing effects of drugs. The present study used behavioral and neurochemical techniques to study the interaction of cannabinoid receptors and nicotine pharmacology. In a locomotor activity experiment in rats, the CB(1)/CB(2) cannabinoid receptor agonist WIN-55,212-2 (0.28-2.8 mg/kg) attenuated nicotine (0.4 mg/kg)-induced hyperactivity, but did not alter nicotine (1.0 mg/kg)-induced hypoactivity. In contrast, the selective CB(1) cannabinoid receptor antagonist SR-141716A (1.0 mg/kg) diminished nicotine-induced hypoactivity, but did not alter nicotine-induced hyperactivity. In a neurochemical experiment, rat striatal slices preloaded with [(3)H]dopamine were superfused with WIN-55,212-2 or SR-141716A. A high concentration (100 microM) of WIN-55,212-2 evoked [(3)H]overflow, but this effect was not blocked by the cannabinoid receptor antagonist AM-251. SR-141716A did not evoke [(3)H]overflow, and neither WIN-55,212-2 nor SR-141716A altered nicotine-evoked [(3)H]overflow. Overall, these results indicate a behavioral interaction between cannabinoid receptors and nicotine pharmacology. Likely, WIN-55,212-2 and SR-141716A block nicotine-induced changes in behavior through an indirect mechanism, such as alteration in endocannabinoid regulation of motor circuits, rather than directly through blockade of nicotinic acetylcholine receptors.

Non-nicotinic therapies for smoking cessation

Cigarette smoking is the primary cause of numerous preventable diseases; as such, the goals of smoking cessation are both to reduce health risks and to improve the quality of life. Currently, the first-line smoking cessation therapies include nicotine replacement products and bupropion. The nicotinic receptor partial agonist varenicline has recently been approved by the FDA for smoking cessation. A newer product currently under development and seeking approval by the FDA are nicotine vaccines. Clonidine and nortriptyline have demonstrated some efficacy but side effects may limit their use to second-line therapeutic products. Other therapeutic drugs that are under development include rimonabant, mecamylamine, monoamine oxidase inhibitors, and dopamine receptor D3 antagonists. Inhibitors of nicotine metabolism are also promising candidates for smoking reduction and cessation. In conclusion, promising new therapeutic products are emerging and they will provide smokers additional options to assist in achieving smoking cessation.

Dopamine D3receptor ligands for the treatment of tobacco dependence

This review considers the potential use of the dopamine D(3) receptor (DRD3) as a novel therapeutic target for the treatment of tobacco dependence. Among the 5 dopamine receptors identified, the DRD3 is located in the nucleus accumbens, ventral tegmental area and amygdala: 3 brain structures that are implicated in the motivational control of drug-seeking behaviour and drug-conditioning processes. Although it has been proposed that modulating dopamine transmission would be effective in the treatment of drug dependence, no validation has been provided in humans so far. Several highly selective DRD3 ligands have recently been evaluated in preclinical models of drug dependence. These ligands act as DRD3 antagonists in vivo and are able to decrease the motivation to take various drugs of abuse and reduce the influence of associated drug-conditioned behaviour. Of note is that these effects have been found with nicotine-seeking behaviour and nicotine relapse in rodents, suggesting a potential use of these ligands for the treatment of tobacco smokers. In contrast to nicotine replacement therapy, varenicline and bupropion (which are currently used for the treatment of smokers), DRD3 antagonists do not seem to produce nicotine-like effects in experimental animals and, therefore, may not substitute for nicotine or alleviate nicotine withdrawal symptoms in human smokers. This behavioural profile, which was also reported recently with cannabinoid CB(1) receptor antagonists, may result from effects on specific brain pathways that express DRD3 receptors and are involved in relapse and conditioning processes. These preclinical studies suggest that the clinical evaluation of DRD3 ligands should be performed with clinical trials designed specifically to evaluate the relapse phenomena.

Differential involvement of the endocannabinoid system in short- and long-term expression of incentive learning supported by nicotine in rats

RATIONALE

We previously reported that the CB1 cannabinoid receptor antagonist, rimonabant, impaired the acquisition and the short-term (24 h), but not long-term (3 weeks), expression of conditioned place preference (CPP) induced by nicotine in rats.

OBJECTIVE

To assess the time interval of efficacy of a single pretest injection of rimonabant to abolish nicotine-CPP, and the effects of chronic CB1 receptor blockade on long-term expression of nicotine-CPP.

MATERIALS AND METHODS

Wistar rats were conditioned to nicotine (0.06 mg/kg, subcutaneous) using an unbiased one-compartment procedure. Two test sessions were conducted 24 h (without injection) and 1, 2, or 3 weeks later. Rimonabant (3 mg/kg, intraperitoneal) or vehicle was administered daily between the two test sessions. In addition, the CB1-stimulated [(35)S]GTP-gamma-S binding was assessed in rats from the 3-week experiment.

RESULTS

The capacity of a single injection of rimonabant (3 mg/kg, 30 min pretest) to block the expression of nicotine-CPP disappeared within 1 week after conditioning. Daily administrations of rimonabant for 6, 13, or 20 days post-acquisition did not impair nicotine-CPP but allowed an additional pretest injection of rimonabant to retain its capacity to abolish long-term expression of nicotine-CPP. The CB1 receptor-mediated G-protein signaling was not altered in various brain areas of rats given rimonabant for 3 weeks.

CONCLUSIONS

The endocannabinoid system is essential to the expression of nicotine-CPP during less than 1 week after conditioning but not later. However, endocannabinoid-dependent mechanisms are critically involved in the development of the neuroadaptive changes responsible for the shift from CB1-dependent to CB1-independent expression of nicotine incentive learning.

Design, synthesis, and biological evaluation of new 1,8-naphthyridin-4(1H)-on-3-carboxamide and quinolin-4(1H)-on-3-carboxamide derivatives as CB2 selective agonists

On the basis of docking studies carried out using the recently published cannabinoid receptor models,35 new 1,8-naphthyridin-4(1H)-on-3-carboxamide and quinolin-4(1H)-on-3-carboxamide derivatives were designed, synthesized, and tested for their affinities toward the cannabinoid CB1 and CB2 receptors. Compound 10, which presented p-fluorobenzyl and carboxycycloheptylamide substituents bound in the 1 and 3 positions of the 1,8-naphthyiridine-4-one nucleus, showed a high CB2 affinity with a Ki of 1.0 nM. The substitution of the naphthyridine-4-one nucleus with the quinoline-4-one system determined a general increase in CB2 affinity. In particular, the N-cyclohexyl-7-chloro-1-(2-morpholin-4-ylethyl)quinolin-4(1H)-on-3-carboxamide (40) possessed a remarkable affinity, with Ki of 3.3 nM, which was also accompanied by a high selectivity for the CB2 receptor (Ki(CB1)/Ki(CB2) ratio greater than 303). Moreover, the [35S]GTPgamma binding assay and functional studies on human basophils indicated that the 1,8-naphthyridin-4(1H)-on-3-carboxamide derivatives behaved as CB1 and CB2 receptor agonists.

Developments in pharmacotherapy for tobacco dependence: past, present and future

In the mid-1970s there were no effective pharmacological treatments for tobacco dependence. The invention of nicotine gum was a major treatment advance and also greatly helped our understanding of the nature of tobacco dependence. There are now eight effective pharmacotherapies (nicotine gum, patch, nasal spray, inhaler, lozenge/tablet, bupropion, nortriptyline and clonidine) available to aid smoking cessation. Other non-nicotine agents that show promise are under investigation, including glucose, rimonabant, selegiline and varenicline. Greater knowledge of the mechanisms of action of the effective non-nicotine agents should lead to better understanding of the nature of tobacco dependence. Future research into optimal treatments should examine long-term combination pharmacotherapy combined with improved psychosocial support that is partly designed to enhance medication compliance. In addition, there is a need for studies designed to evaluate the efficacy of pharmacotherapies in populations such as youth, pregnant smokers and smokers with co-occurring mental health problems.

The endocannabinoid system as an emerging target of pharmacotherapy

The recent identification of cannabinoid receptors and their endogenous lipid ligands has triggered an exponential growth of studies exploring the endocannabinoid system and its regulatory functions in health and disease. Such studies have been greatly facilitated by the introduction of selective cannabinoid receptor antagonists and inhibitors of endocannabinoid metabolism and transport, as well as mice deficient in cannabinoid receptors or the endocannabinoid-degrading enzyme fatty acid amidohydrolase. In the past decade, the endocannabinoid system has been implicated in a growing number of physiological functions, both in the central and peripheral nervous systems and in peripheral organs. More importantly, modulating the activity of the endocannabinoid system turned out to hold therapeutic promise in a wide range of disparate diseases and pathological conditions, ranging from mood and anxiety disorders, movement disorders such as Parkinson's and Huntington's disease, neuropathic pain, multiple sclerosis and spinal cord injury, to cancer, atherosclerosis, myocardial infarction, stroke, hypertension, glaucoma, obesity/metabolic syndrome, and osteoporosis, to name just a few. An impediment to the development of cannabinoid medications has been the socially unacceptable psychoactive properties of plant-derived or synthetic agonists, mediated by CB(1) receptors. However, this problem does not arise when the therapeutic aim is achieved by treatment with a CB(1) receptor antagonist, such as in obesity, and may also be absent when the action of endocannabinoids is enhanced indirectly through blocking their metabolism or transport. The use of selective CB(2) receptor agonists, which lack psychoactive properties, could represent another promising avenue for certain conditions. The abuse potential of plant-derived cannabinoids may also be limited through the use of preparations with controlled composition and the careful selection of dose and route of administration. The growing number of preclinical studies and clinical trials with compounds that modulate the endocannabinoid system will probably result in novel therapeutic approaches in a number of diseases for which current treatments do not fully address the patients' need. Here, we provide a comprehensive overview on the current state of knowledge of the endocannabinoid system as a target of pharmacotherapy.

Alcohol-related genes: contributions from studies with genetically engineered mice

Since 1996, nearly 100 genes have been studied for their effects related to ethanol in mice using genetic modifications including gene deletion, gene overexpression, gene knock-in, and occasionally by studying existing mutants. Nearly all such studies have concentrated on genes expressed in brain, and the targeted genes range widely in their function, including most of the principal neurotransmitter systems, several neurohormones, and a number of signaling molecules. We review 141 published reports of effects (or lack thereof) of 93 genes on responses to ethanol. While most studies have focused on ethanol self-administration and reward, and/or sedative effects, other responses studied include locomotor stimulation, anxiolytic effects, and neuroadaptation (tolerance, sensitization, withdrawal). About 1/4 of the engineered mutations increase self-administration, 1/3 decrease it, and about 40% have no significant effect. In many cases, the effects on self-administration are rather modest and/or depend on the specific experimental procedures. In some cases, genes in the background strains on which the mutant is placed are important for results. Not surprisingly, review of the systems affected further supports roles for serotonin, gamma-aminobutyric acid, opioids and dopamine, all of which have long been foci of alcohol research. Novel modulatory effects of protein kinase C and G protein-activated inwardly rectifying K+ (GIRK) channels are also suggested. Some newer research with cannabinoid systems is promising, and has led to ongoing clinical trials.

Anandamide administration alone and after inhibition of fatty acid amide hydrolase (FAAH) increases dopamine levels in the nucleus accumbens shell in rats

Although endogenous cannabinoid systems have been implicated in the modulation of the rewarding effects of abused drugs and food, little is known about the direct effects of endogenous ligands for cannabinoid receptors on brain reward processes. Here we show for the first time that the intravenous administration of anandamide, an endogenous ligand for cannabinoid receptors, and its longer-lasting synthetic analog methanandamide, increase the extracellular dopamine levels in the nucleus accumbens shell of awake, freely moving rats, an effect characteristic of most drugs abused by humans. Anandamide produced two distinctly different effects on dopamine levels: (1) a rapid, transient increase that was blocked by the cannabinoid CB1 receptor antagonist rimonabant, but not by the vanilloid VR1 receptor antagonist capsazepine, and was magnified and prolonged by the fatty acid amide hydrolase (FAAH) enzyme inhibitor, URB597; (2) a smaller delayed and long-lasting increase, not sensitive to CB1, VR1 or FAAH blockade. Both effects were blocked by infusing either tetrodotoxin (TTX, 1 microm) or calcium-free Ringer's solution through the microdialysis probe, demonstrating that they were dependent on the physiologic activation of dopaminergic neurotransmission. Thus, these results indicate that anandamide, through the activation of the mesolimbic dopaminergic system, participates in the signaling of brain reward processes.

The Effects of Glutamate and GABA Receptor Antagonists on Nicotine-induced Neurotransmitter Changes in Cognitive Areas*

In the present study, we tested the effects of glutamate and GABA receptor antagonists on nicotine-induced neurotransmitter changes in the hippocampal (dorsal and ventral) and cortical (medial temporal and prefrontal) brain areas of conscious freely moving rats via microdialysis. Both the antagonists and nicotine were administered intracerebrally. The antagonists tested were NMDA, AMPA-kainate, and metabotropic glutamate receptor subtype antagonists (MK801, CNQX, and LY 341495, respectively) and GABA(A) and GABA(B) receptor subtype antagonists (bicuculline and hydroxysaclofen, respectively). We assayed nicotine-induced changes in dopamine (DA), norepinephrine (NE), serotonin (5-HT), and their metabolites. We found with the antagonists, both decreases and increases in nicotine-induced neurotransmitter responses. In the presence of nicotine all the antagonists (except LY 341495) caused a decrease in DA levels in the regions tested. NE levels were decreased in the cortex by all antagonists. In the hippocampus, GABA antagonists decreased NE levels, as did the metabotropic glutamate antagonist, LY 341495, while the other glutamate antagonists increased NE levels. The results of the 5-HT assay were more variable and dependent on the region and antagonist examined; increases were found slightly more often than decreases. The changes in metabolites were not often parallel with changes in their associated neurotransmitters, indicating that the antagonists also affect the metabolism of the neurotransmitters. The effect of the antagonists in the absence of nicotine was mostly to decrease the level of neurotransmitters, although increases were seen in a few cases. The results suggest that the excitatory glutamatergic- and inhibitory GABAergic-amino acid receptors are both involved in mediating nicotine-induced neurotransmitter responses, and their inhibitory or stimulatory effects are receptor subtype and brain region dependent.

Leptin resistance and obesity

The prevalence of obesity, and the human and economic costs of the disease, creates a need for better therapeutics and better understanding of the physiological processes that balance energy intake and energy expenditure. Leptin is the primary signal from energy stores and exerts negative feedback effects on energy intake. In common obesity, leptin loses the ability to inhibit energy intake and increase energy expenditure; this is termed leptin resistance. This review discusses the evidence in support of leptin resistance in mouse models and humans and the possible mechanisms of leptin resistance.

SR147778, a CB1 cannabinoid receptor antagonist, suppresses ethanol preference in chronically alcoholized Wistar rats

This study investigated the effect of the new CB1 cannabinoid receptor antagonist, SR147778, on ethanol preference in chronically alcoholized Wistar rats. In study 1, SR147778, at doses of 0.3, 1, or 10 mg/kg/day (mg/kg/d) intraperitonealy (ip), was administered during chronic pulmonary ethanol intoxication for 30 days. The rats were then exposed to a two-bottle choice (ethanol 10% v/v vs. water) for at least 30 days. Neither 0.3 nor 1 mg/kg/d had any effect on ethanol preference. In contrast, the high dose induced a significant transient increase in ethanol intake between days 6 and 10. In study 2, SR147778, at doses of 0.3, 1, or 10 mg/kg/d ip, was administered during the free-choice period after chronic alcoholization. Both ethanol preference and intake were significantly reduced only for 1 and 10 mg/kg/d. These results reinforce the hypothesis that the cannabinoid CB1 receptor is part of the neural substrate mediating alcohol intake and the motivational properties of alcohol. When these results are compared with those obtained with SR141716 (Rimonabant) on ethanol preference, we observed that (1) coadministration of 10 mg/kg/d SR147778 during chronic alcoholization induced a shorter transient increase of ethanol intake than Rimonabant and (2) SR147778 treatment during the free-choice period at doses of 1 and 10 mg/kg/d decreased ethanol intake more dramatically than SR141716 which, furthermore, continued for the duration of the free choice.

Neuropharmacology and potential efficacy of new treatments for tobacco dependence

This review considers some of the novel therapies that are under development for the treatment of tobacco dependence, outlines their efficacy in clinical studies and explains their mechanisms of action in terms of contemporary theories for the psychobiology of the dependence. It focuses on three treatments with differing mechanisms of action that are at different stages of clinical development. The first is varenicline, a partial agonist at the alpha4beta2 nicotinic receptors, which are thought to play a central role in the addiction to nicotine. Preclinically, this drug mimics the effects of nicotine on dopamine (DA) release in the nucleus accumbens when given alone but attenuates this response to a subsequent nicotine challenge and reduces nicotine self administration. Very encouraging results have been seen in the five clinical studies that have been reported with this drug. The second compound, rimonabant, is a cannabinoid CB1 receptor antagonist. Preclinically, this compound reduces nicotine self administration, DA turnover in nucleus accumbens and attenuates reinstatement of nicotine-seeking behaviour. Clinically, the drug is well tolerated but its effects on smoking cessation are equivocal. However, it has the valuable additional property of inhibiting post-cessation weight gain. Nicotine 'vaccines' are the final group of treatments considered, which involves raising antibodies in the blood that limit the amount of nicotine that penetrates into the brain, thereby reducing the psychopharmacological responses to the drug. The vaccines also reduce DA turnover in nucleus accumbens and reinstatement of nicotine-seeking behaviour after nicotine readministration. The three vaccines discussed are well tolerated and show signs of good efficacy; however, the increase in antibody titre, evoked by the treatment, shows significant inter-individual variation and is generally short lived. Thus, although this approach may provide a valuable aid to smoking cessation, it seems unlikely that it can be used for primary prevention.

Evaluation of the role of nicotinic acetylcholine receptor subtypes and cannabinoid system in the discriminative stimulus effects of nicotine in rats

Male Wistar rats were trained to discriminate (-)-nicotine (0.4 mg/kg) from saline under a two-lever, fixed-ratio 10 schedule of water reinforcement. During test sessions the following drugs were coadministered with saline (substitution studies) or nicotine (0.025-0.4 mg/kg; combination studies): the alpha4beta2 nicotinic acetylcholine receptor subtype antagonist dihydro-beta-erythroidine (DHbetaE), the non-selective nicotinic acetylcholine receptor subtype antagonist mecamylamine, the alpha7 nicotinic acetylcholine receptor subtype antagonist methyllycaconitine (MLA), the alpha4beta2 nicotinic acetylcholine receptor subtype agonist 5-iodo-3-(2(S)-azetidinylmethoxy)pyridine (5-IA), the cannabinoid CB1 receptor antagonist/partial agonist rimonabant, the cannabinoid CB2 receptor antagonist N-[(1S)-endo-1,3,3-trimethylbicyclo-[2.2.1]heptan-2-yl]5-(4-chloro-3-methyl-phenyl)-1-(4-methybenzyl)pyrazole-3-carboxamide (SR 144528), the cannabinoid CB1/2 receptor agonists (-)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)-phenyl]-trans-4-(3-hydroxy-propyl)cyclohexanol (CP 55,940) or R(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]-pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-(1-naphthalenyl)-methanone mesylate (WIN 55,212-2), the endogenous cannabinoid agonist and non-competitive alpha7 nicotinic acetylcholine receptor subtype antagonist anandamide, the anandamide uptake and fatty acid amide hydrolase inhibitor N-(4-hydroxyphenyl)-5Z,8Z,11Z,14Z-eicosatetraenamide (AM-404), the fatty acid amide hydrolase inhibitor cyclohexylcarbamic acid 3'-carbamoyl-biphenyl-3-yl ester (URB 597), AM-404+anandamide or URB 597+anandamide. 5-IA (0.01 mg/kg) fully substituted for nicotine, while other drugs were inactive. In combination studies, DHbetaE and mecamylamine dose-dependently attenuated the discriminative stimulus effects of nicotine and the full substitution of 5-IA, while MLA, rimonabant, SR 144528, CP 55,940, WIN 55,212-2, and URB 597 did not alter the nicotine cue. Pretreatment with AM-404+anandamide or URB 597+anandamide weakly enhanced nicotine-lever responding. Our pharmacological analyses demonstrates that the expression of nicotine discrimination is under the control of nicotinic acetylcholine receptor subtypes composed of alpha4beta2 (but not of alpha7) subunits. Furthermore, we excluded the involvement of either cannabinoid CB1 and CB2 receptors or increases in the endocannabinoid tone in the nicotine discrimination.

Involvement of the endocannabinoid system in drug addiction

Recent studies have shown that the endocannabinoid system is involved in the common neurobiological mechanism underlying drug addiction. This system participates in the primary rewarding effects of cannabinoids, nicotine, alcohol and opioids, through the release of endocannabinoids in the ventral tegmental area. Endocannabinoids are also involved in the motivation to seek drugs by a dopamine-independent mechanism, demonstrated for psychostimulants and opioids. The endocannabinoid system also participates in the common mechanisms underlying relapse to drug-seeking behaviour by mediating the motivational effects of drug-related environmental stimuli and drug re-exposure. In agreement, clinical trials have suggested that the CB(1) cannabinoid antagonist rimonabant can cause smoking cessation. Thus, CB(1) cannabinoid antagonists could represent a new generation of compounds to treat drug addiction.

Role of the cannabinoid system in the effects induced by nicotine on anxiety-like behaviour in mice

RATIONALE

Acute behavioural effects and motivational responses induced by nicotine can be modulated by the endocannabinoid system supporting the existence of a physiological interaction between these two systems.

OBJECTIVES

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

METHODS

Animals were only exposed once to nicotine. The acute administration of low (0.05) or high (0.8 mg/kg, s.c.) doses of nicotine produced opposite effects in the elevated plus-maze, i.e. anxiolytic- and anxiogenic-like responses, respectively. The effects of the pretreatment with the CB1 cannabinoid receptor antagonist, rimonabant (0.25, 0.5 and 1 mg/kg, i.p.), and the cannabinoid agonist, delta9-tetrahydrocannabinol (delta9-THC, 0.1 mg/kg, ip), were evaluated on the anxiolytic- and anxiogenic-like responses induced by nicotine.

RESULTS

Rimonabant completely abolished nicotine-induced anxiolytic-like effects and increased the anxiogenic-like responses of nicotine, suggesting an involvement of CB1 receptors in these behavioural responses. On the other hand, delta9-THC failed to modify nicotine anxiolytic-like responses but attenuated its anxiogenic-like effects. In addition, the association of non-effective doses of delta9-THC and nicotine produced clear anxiolytic-like responses.

CONCLUSIONS

These results demonstrate that the endogenous cannabinoid system is involved in the regulation of nicotine anxiety-like behaviour in mice and provide new findings to support the use of cannabinoid antagonists in the treatment of tobacco addiction.

Rimonabant: a cannabinoid receptor blocker for the treatment of metabolic and cardiovascular risk factors

AIMS

The endocannabinoid system modulates synaptic neurotransmission centrally and peripherally and is involved in the brain pathways concerned with addiction, central regulation of body weight and adipose tissue function. The system is overactivated in animal models of obesity and nicotine use. This review discusses the role of rimonabant, a cannabinoid receptor 1 blocker, which has undergone Phase III clinical testing, in the treatment of obesity and tobacco dependence.

DATA SYNTHESIS

Results of Phase III clinical trials have shown that rimonabant has promising efficacy in the treatment of obesity, dyslipidaemia and diabetes associated with obesity, in preventing weight gain following smoking cessation, and possibly in smoking cessation. No critical problems with the tolerance and safety of the compound have appeared in studies to date.

CONCLUSION

Rimonabant may prove to be a useful aid in the treatment of the most widespread cardiometabolic risk factors.

Nicotine as a typical drug of abuse in experimental animals and humans

RATIONALE AND BACKGROUND

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 reinforcing effects of nicotine often are difficult to demonstrate directly in controlled laboratory studies with animal or human subjects.

OBJECTIVE

To review the major findings obtained with various procedures developed to study dependence-related behavioral effects of nicotine in experimental animals and humans, i.e., drug self-administration, conditioned place preference, subjective reports of nicotine effects and nicotine discrimination, withdrawal signs, and ratings of drug withdrawal.

RESULTS

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 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 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. Further experiments are also needed to delineate the role other chemical substances inhaled along with nicotine in tobacco smoke play in sustaining smoking behavior.

CONCLUSION

Nicotine acts as a typical drug of abuse in experimental animals and humans.

Nicotine and non-nicotine smoking cessation pharmacotherapies

Cigarette smoking remains an important risk factor for premature cardiovascular disease and its complications. There are clear benefits of cigarette smoking cessation on the rate of clinical outcomes, and in addition to behavioral therapies, various pharmacologic strategies have been developed to help achieve this goal. First-line therapies include nicotine replacement and/or bupropion. Second-line treatments include clonidine and nortriptyline. Additional treatment strategies, with less proven efficacy, include monoamine oxidase inhibitors, selective serotonin reuptake inhibitors, opioid receptor antagonists, bromocriptine, antianxiety drugs, nicotinic receptor antagonists (eg, mecamylamine), and glucose tablets. Various approaches under investigation include the use of partial nicotine agonists (eg, varenicline), inhibitors of the hepatic P-450 enzyme (eg, methoxsalen), cannaboid-1 receptor antagonists (eg, rimonabant), and nicotine vaccines.

Hypothalamic regulatory pathways and potential obesity treatment targets

With an ever-growing population of obese people as well as comorbidities associated with obesity, finding effective weight loss strategies is more imperative than ever. One of the challenges in curbing the obesity crisis is designing successful strategies for long-term weight loss and weight-loss maintenance. Currently, weight-loss strategies include promotion of therapeutic lifestyle changes (diet and exercise), pharmacological therapy, and bariatric surgery. This review focuses on several pharmacological targets that activate central nervous system pathways that normally limit food intake and body weight. Though it is likely that no single therapy will prove effective for everyone, this review considers several recent pre-clinical targets, and several compounds that have been in human clinical trials.

CANNABINOID RECEPTORS AS THERAPEUTIC TARGETS

CB1 and CB2 cannabinoid receptors are the primary targets of endogenous cannabinoids (endocannabinoids). These G protein-coupled receptors play an important role in many processes, including metabolic regulation, craving, pain, anxiety, bone growth, and immune function. Cannabinoid receptors can be engaged directly by agonists or antagonists, or indirectly by manipulating endocannabinoid metabolism. In the past several years, it has become apparent from preclinical studies that therapies either directly or indirectly influencing cannabinoid receptors might be clinically useful. This review considers the components of the endocannabinoid system and discusses some of the most promising endocannabinoid-based therapies.

Is there a role for the endocannabinoid system in the etiology and treatment of melancholic depression?

With advances in basic and clinical neuroscience, many gaps have appeared in the traditional monoamine theory of depression that have led to reformulation of the hypotheses concerning the neurobiology of depression. The more recent hypotheses suggest that melancholic depression is characterized by central glucocorticoid resistance that results in hypercortisolemia, which in turn leads to down-regulation of neurotrophins and subsequent neurodegeneration. Examining the neurobiology of depression from this perspective suggests that the endocannabinoid system may play a role in the etiology of melancholic depression. Specifically, pharmacological and genetic blockade of the cannabinoid CB1 receptor induces a phenotypic state that is analogous to melancholic depression, including symptoms such as reduced food intake, heightened anxiety, increased arousal and wakefulness, deficits in extinction of aversive memories and supersensitivity to stress. These similarities between melancholic depression and an endocannabinoid deficiency become more interesting in light of recent findings that endocannabinoid activity is down-regulated by chronic stress and possibly increased by some antidepressant regimens. We propose that an endocannabinoid deficiency may underlie some of the symptoms of melancholic depression, and that enhancement of this system may ultimately be a novel form of pharmacotherapy for treatment-resistant depression.

Current evidence supporting a role of cannabinoid CB1 receptor (CB1R) antagonists as potential pharmacotherapies for drug abuse disorders

Since the discovery of the cannabinoid CB1 receptor (CB1R) in 1988, and subsequently of the CB2 receptor (CB2R) in 1993, there has been an exponential growth of research investigating the functions of the endocannabinoid system. The roles of CB1Rs have been of particular interest to behavioral pharmacologists because of their selective presence within the central nervous system (CNS) and because of their association with brain-reward circuits involving mesocorticolimbic dopamine systems. One potential role that has become of considerable recent focus is the ability of CB1Rs to modulate the effects of drugs of abuse. Many drugs of abuse elevate dopamine levels, and the ability of CB1R antagonists or inverse agonists to attenuate these elevations has suggested their potential application as pharmacotherapies for treating drug abuse disorders. With the identification of the selective CB1R antagonist, SR141716, in 1994, and its subsequent widespread availability, there has been a rapid expansion of research investigating its ability to modulate the effects of drugs of abuse. The preliminary clinical reports of its success in retarding relapse in tobacco users have accelerated this expansion. This report critically reviews preclinical and clinical studies involving the ability of CB1R antagonists to attenuate the effects of drugs of abuse, while providing an overview of the neuroanatomical and neurochemical points of contact between the endocannabinoid system and systems mediating abuse-related effects.

Lipids and drugs of abuse

Drug abuse continues to take an enormous economic and social toll on the world. Among the costs are reduced productivity, increased need for medical services and stress on families. Treatments that allow affected individuals to reduce compulsive drug use are lacking and novel approaches to their development will likely come from increased understanding of the consequences of chronic exposure to reinforcing drugs. The purpose of this review is to explore the role of lipids in drug abuse and to present a rationale for an increased focus on the interactions between drugs of abuse and lipids in the brain. Small molecular weight lipids function as neuromodulators in the brain and, as such, play a role in the synaptic plasticity that occurs following exposure to drugs of abuse. In addition, the membrane lipid bilayer consists of lipid subdomains and emerging evidence suggests that protein function can be altered by transient associations with these subdomains. Finally, lipidomics is a very new field devoted to the exploration of changes in cellular lipid constituents during phenotypic alterations. Enhanced research in all of these areas will likely provide useful insights into and, perhaps, therapeutic targets for the treatment of drug abuse.

Effect of the cannabinoid CB1 receptor antagonist SR-141716A on ethanol self-administration and ethanol-seeking behaviour in rats

RATIONALE

It has been suggested that endocannabinoid mechanisms are involved in the control of ethanol consumption.

OBJECTIVES

The aims of the present study were (1) to evaluate the role of the endocannabinoid system in the control of operant ethanol self-administration and in the reinstatement of ethanol seeking, when induced by stress or conditioned stimuli and (2) to offer new insights on the specificity of such a role.

METHODS

Rats were administered intraperitoneally with the selective cannabinoid CB1 receptor antagonist, SR-141716A, 30 min before operant self-administration or reinstatement sessions. Two schedules of reinforcement, the fixed-ratio 1 (FR1) and the progressive ratio (PR), were used to study 10% (w/v) alcohol and 5.0% sucrose self-administration. NaCl (2% w/v) intake in sodium-depleted rats was studied only under the FR1 program.

RESULTS

Treatment with SR-141716A (0.3-3.0 mg/kg) significantly attenuated FR1 alcohol self-administration and lowered the break point for ethanol under PR. SR-141716A also markedly inhibited the reinstatement of alcohol seeking elicited by presentation of cues predictive of drug availability. Conversely, the cannabinoid antagonist did not prevent the reinstatement of alcohol seeking induced by foot-shock stress. Lever pressing for sucrose under FR1 and PR schedules was also significantly decreased by SR-141716A treatment, whereas the drug modestly and only at the highest dose decreased 2% NaCl self-administration.

CONCLUSIONS

Results emphasize that endocannabinoid mechanisms play a major role in the control of ethanol self-administration and in the reinstatement of conditioned ethanol seeking. However, these effects extend to the control of operant behaviours motivated by natural rewards (i.e. sucrose). On the other hand, SR-141716A only weakly reduces NaCl self-administration in sodium-depleted rats, in which salt intake is largely controlled by homeostatic mechanisms. Overall, these observations demonstrate that the inhibition of operant behaviour following blockade of CB1 receptors by SR-141716A is linked to a reduction of reward-related responding and is not related to drug-induced motor deficits.

The relationship of in vivo central CB1 receptor occupancy to changes in cortical monoamine release and feeding elicited by CB1 receptor antagonists in rats

RATIONALE

Cannabinoid type 1 (CB(1)) receptor antagonists are reportedly effective in reducing food intake both preclinically and clinically. This may be due in part to their effects on monoamine release in the brain. The level of central CB(1) receptor occupancy underlying these neurobiological effects is unclear.

OBJECTIVES

We explored the relationship between in vivo CB(1) receptor occupancy in the frontal cortex and changes in both monoamine release in the medial prefrontal cortex (mPFC) and feeding behavior in rats in response to two orally administered CB(1) receptor antagonists presently in clinical trials, SR141716A (rimonabant) and SLV319.

METHODS

CB(1) receptor occupancy was measured using [(3)H] SR141716A, and these occupancies were related to potencies to mediate increases in dopamine (DA) and norepinephrine (NE) release measured with microdialysis and decreases in consumption of a highly palatable diet (HP).

RESULTS

High receptor occupancy levels (>65%) were required to detect increases in monoamine release that were achieved with 3 and 10 mg/kg of SR141716A and 10 mg/kg of SLV319 for DA and 10 mg/kg of SR141716A for NE. Decreases in HP consumption were seen at occupancies higher than 65% for SR141716A that were achieved with 3 and 10 mg/kg. In contrast, decreases in HP consumption were seen at relatively low CB(1) receptor occupancies (11%) for SLV319.

CONCLUSIONS

The occupancy method described here is an effective tool for interrelating central CB(1) receptor occupancy with neurobiological actions of CB(1) receptor antagonists and for furthering our understanding of the role of CB(1) receptors in central nervous system physiology and pathology.

Analysis of the endocannabinoid system by using CB1 cannabinoid receptor knockout mice

The endocannabinoid system has been involved in the control of several neurophysiological and behavioural responses. To date, three lines of CB1 knockout mice have been established independently in different laboratories. This chapter reviews the main results obtained with these lines of CB1 knockout mice in several physiological responses that have been previously related to the activity of the endocannabinoid system. Studies using CB1 knockout mice have demonstrated that this receptor participates in the control of several behavioural responses including locomotion, anxiety- and depressive-like states, cognitive functions such as memory and learning processes, cardiovascular responses and feeding. Furthermore, the CB1 cannabinoid receptor is involved in the control of pain by acting at peripheral, spinal and supraspinal levels. The involvement of the CB1 cannabinoid receptor in the behavioural and biochemical processes underlying drug addiction has also been investigated. These CB1 knockouts have provided new findings to clarify the interactions between cannabinoids and the other drugs of abuse such as opioids, psychostimulants, nicotine and ethanol. Recent studies have demonstrated that endocannabinoids can function as retrograde messengers, modulating the release of different neurotransmitters, including opioids, gamma-aminobutyric acid (GABA), and cholecystokinin (CCK), which could explain some of the responses observed after the stimulation of the CB1 cannabinoid receptor. This review provides an update of the apparently controversial data reported in the literature using the three different lines of CB1 knockout mice, which seem to be mainly due to the use of different experimental procedures rather than any constitutive alteration in these lines of knockouts.

Cannabinoids, opioids and eating behavior: the molecular face of hedonism?

Obesity represents nowadays one of the most devastating health threats. Published reports even project a decline in life expectancy of US citizens due to the rapidly increasing prevalence of obesity. This alarming increase is intimately linked with recent changes of environment and lifestyle in western countries. In this context, the rewarding or even addictive properties of popular food may represent one of the most serious obstacles to overcome for an effective anti-obesity therapy. Therefore, in addition to molecular networks controlling energy homeostasis, now researchers are starting to define central nervous mechanisms governing hedonic and addictive components of food intake. A recently emerging body of data suggests that the endogenous cannabinoid and opioid systems both represent key circuits responding to the rewarding value of food. This review focuses on the role of these two systems for the homeostatic and hedonic aspects of eating behavior and includes their anatomical and functional interactions. Independent from the degree to which eating can be considered an addiction, cannabinoid and opioid receptor antagonists are promising anti-obesity drugs, since they are targeting both hedonic and homeostatic components of energy balance control.

Ethanol does not affect discriminative-stimulus effects of nicotine in rats

The effects of ethanol were evaluated in rats trained to discriminate 0.4 mg/kg of nicotine from saline under a fixed-ratio 10 schedule of food delivery. Ethanol (0.1-1 g/kg, i.p.) did not produce any nicotine-like discriminative effects and did not produce any shift in the dose-response curve for nicotine discrimination. Thus, the ability to discriminate nicotine's effects does not appear to be altered by ethanol administration. However, the high dose of 1 g/kg ethanol, given either alone or in combination with nicotine, markedly depressed food-maintained responding. This later effect was associated in some rats with an attenuation of the discriminative-stimulus effects of the training dose of nicotine. This suggests that previous reports of increased tobacco smoking following ethanol consumption in humans are connected, in some way, with an increase in motivation to consume nicotine that is produced by ethanol, rather than with a decrease in the subjective response to nicotine.

Nicotine use in schizophrenia: the self medication hypotheses

The behavioural and cognitive effects of nicotine in schizophrenia have received much interest in recent years. The rate of smoking in patients with schizophrenia is estimated to be two- to four-fold the rate seen in the general population. Furthermore such patients favour stronger cigarettes and may also extract more nicotine from their cigarettes than other smokers. The question has been raised whether the widespread smoking behaviour seen in this patient group is in fact a manifestation of a common underlying physiology, and that these patients smoke in an attempt to self-medicate. We present an overview of the explanations for elevated rates of smoking in schizophrenia, with particular emphasis on the theories relating this behaviour to sensory gating and cognitive deficits in this disorder that have been viewed as major support for the self-medication hypotheses.

1-Benzhydryl-3-phenylurea and 1-Benzhydryl-3-phenylthiourea Derivatives: New Templates among the CB1 Cannabinoid Receptor Inverse Agonists

New 1-benzhydryl-3-phenylurea derivatives and their 1-benzhydryl-3-phenylthiourea isosteres were synthesized and evaluated for their human CB1 and CB2 cannabinoid receptor affinity. These compounds proved to be selective CB1 cannabinoid receptor ligands, acting as inverse agonists in a [35S]-GTPgammaS assay. The affinity of 3,5,5'-triphenylimidazolidine-2,4-dione and 3,5,5'-triphenyl-2-thioxoimidazolidin-4-one derivatives, possessing the 1-benzhydryl-3-phenylurea and 1-benzhydryl-3-phenylthiourea moiety, respectively, was also evaluated. In conclusion, the 1-benzhydryl-3-phenylurea scaffold seems to be a new interesting template of CB1 cannabinoid receptor inverse agonists.

Cannabinoid CB1 receptors are involved in motivational effects of nicotine in rats

RATIONALE

The endocannabinoid system plays a role in mediating the appetitive value of a variety of reinforcing compounds, either natural rewards or drugs of abuse, but little is known about its involvement in the incentive properties of nicotine.

OBJECTIVES

The objective of the study is to evaluate whether activation of CB1 cannabinoid receptors is necessary for the establishment and the short- and long-term expression of nicotine-induced conditioned place preference (CPP). This was studied in rats subjected to an unbiased, one-compartment place conditioning procedure, using the selective CB1 receptor antagonist, rimonabant, as a pharmacological tool.

METHODS

Wistar rats, given previous experience with nicotine in their home cage, were subjected to eight alternating nicotine (0.006-0.6 mg/kg s.c.) and saline pairings with distinct floor textures in an open field and given a test session, with no nicotine injection, in the open field whose floor was covered by two quadrants of the saline-paired texture and two quadrants of the nicotine-paired texture. Rimonabant (0.3-3 mg/kg i.p.) was administered 30 min before each nicotine (0.06 mg/kg) pairing to assess its effect on the establishment of nicotine-CPP. To study the effects of CB1 receptor blockade on short- and long-term expression of nicotine-CPP, rimonabant was administered as a single injection 30 min before the test session, conducted either 24 h, 3 weeks or 12 weeks after the last conditioning session.

RESULTS

Rats developed reliable and robust CPP to the 0.06- and 0.125-mg/kg doses of nicotine. Once established, CPP persisted for at least 12 weeks without additional exposure to nicotine and the test apparatus. Pre-pairing injections of rimonabant (3 mg/kg, but not lower doses) prevented the acquisition of nicotine-CPP, and a single pretest administration of rimonabant (3 mg/kg) abolished the expression of nicotine-CPP when the test session took place 24 h after the last conditioning session. However, rimonabant (3 mg/kg) did not antagonize the expression of nicotine-CPP when the test session was conducted 3 or 12 weeks after the acquisition phase.

CONCLUSIONS

The endocannabinoids are a necessary component in both the perception by rats of the motivational value of nicotine and the short-term capacity of nicotine-paired conditioned stimuli to elicit approach behaviour. In contrast, the acute blockade of CB1 receptors no longer impairs the long-term control of behaviour by nicotine-associated environmental cues. These data provide support to the notion that the blockade of CB1 receptors can oppose tobacco dependence, withdrawal and even relapse, though the time window of efficacy and/or the schedule of administration remain to be established.

Expanding treatment of tobacco dependence

Nicotine dependence is the leading preventable cause of adult morbidity and mortality in the world. New research on the treatment of this disorder ranges from studies evaluating access to treatment to studies elucidating the molecular mechanisms of nicotine addiction. As our understanding of the neurobiology of tobacco addiction grows, the number of potential therapeutic targets by which we can intervene in this pernicious disorder also increases. This paper presents an overview of recent research trends in the treatment of tobacco dependence. We review several novel mechanisms of action that may serve as therapeutic targets for the pharmacologic treatment of tobacco dependence, including drugs that affect monamine oxidase, selective nicotinic receptors, glutamate and gamma-aminobutyric acid receptors, and the endocannabinoid system. For each of these therapeutic targets, we discuss medications in development that affect these pathophysiologic mechanisms.

Rimonabant: The first therapeutically relevant cannabinoid antagonist

The present paper synthetically reviews the multiple experimental lines of evidence indicating the ability of the prototypic cannabinoid CB(1) receptor antagonist, rimonabant (also known as SR 141716), to suppress the reinforcing/rewarding properties of different drugs of abuse, including cocaine, heroin, nicotine and alcohol, in laboratory rodents. This paper also reviews the data demonstrating that rimonabant reduces food intake and body weight in laboratory animals and humans. Taken together, the data reviewed here suggest that rimonabant may constitute a new and potentially effective medication for the treatment of drug addiction and obesity-related disorders.

Lack of CB1 cannabinoid receptor impairs cocaine self-administration

Acute rewarding properties are essential for the establishment of cocaine addiction, and multiple neurochemical processes participate in this complex behavior. In the present study, we used the self-administration paradigm to evaluate the role of CB1 cannabinoid receptors in several aspects of cocaine reward, including acquisition, maintenance, and motivation to seek the drug. For this purpose, both CB1 receptor knockout mice and wild-type littermates were trained to intravenously self-administer cocaine under different schedules. Several cocaine training doses (0.32, 1, and 3.2 mg/kg/infusion) were used in the acquisition studies. Only 25% of CB1 knockout mice vs 75% of their wild-type littermates acquired a reliable operant responding to self-administer the most effective dose of cocaine (1 mg/kg/infusion), and the number of sessions required to attain this behavior was increased in knockout mice. Animals reaching the acquisition criteria were evaluated for the motivational strength of cocaine as a reinforcer under a progressive ratio schedule. The maximal effort to obtain a cocaine infusion was significantly reduced after the genetic ablation of CB1 receptors. A similar result was obtained after the pharmacological blockade of CB1 receptors with SR141716A in wild-type mice. Moreover, the cocaine dose-response curve was flattened in the knockout group, suggesting that the differences observed between genotypes were related to changes in the reinforcing efficacy of the training dose of cocaine. Self-administration for water and food was not altered in CB1 knockout mice in any of the reinforcement schedules used, which emphasizes the selective impairment of drug reinforcement in these knockout mice. Finally, cocaine effects on mesolimbic dopaminergic transmission were evaluated by in vivo microdialysis in these mice. Acute cocaine administration induced a similar enhancement in the extracellular levels of dopamine in the nucleus accumbens of both CB1 knockout and wild-type mice. This work clearly demonstrates that CB1 receptors play an important role in the consolidation of cocaine reinforcement, although are not required for its acute effects on mesolimbic dopaminergic transmission.

The effects of cholinergic and dopaminergic antagonists on nicotine-induced cerebral neurotransmitter changes

In a continuing study of nicotine-induced mechanisms in brain areas associated with cognitive processes, the effects of cholinergic and dopaminergic antagonists on nicotine-induced changes in dopamine, norepinephrine, and serotonin were examined. These effects were measured via in vivo microdialysis in the dorsal and ventral hippocampus and in the prefrontal and medial temporal cortex of conscious, freely moving, adult male rats. Nicotine (0.3 mg/kg, free base) was administered subcutaneously and the antagonists were infused locally via the microdialysis probe. Nicotine alone induced an increase of dopamine and its metabolites in all areas, an increase of norepinephrine in the cortex, and an increase of the norepinephrine metabolite 4-hydroxy-3-methoxy-phenylglycol in all areas. Serotonin was decreased in the hippocampus and increased in the cortex. Nicotine-induced dopamine increases were inhibited by nicotinic (mecamylamine 100 microM, methyllycaconitine 500 microM), muscarinic (atropine 100 microM), and dopaminergic D1 (SCH23390 100 microM) and D2 (eticlopride 100 microM) antagonists, in the hippocampal and cortical areas. In the hippocampal areas, these antagonists had less significant effect on norepinephrine and serotonin. However, in the cortical areas, all antagonists inhibited the nicotine-induced increase of serotonin to varying degrees; and some, primarily nicotinic and dopamine D1 antagonists, inhibited the induced increase of norepinephrine. In the hippocampal and cortical areas, the mechanisms of nicotine-induced dopamine increase seem to be similar, but the mechanisms seem to be different for noradrenergic and serotonergic systems, as shown by the fact that nicotine induces no change in norepinephrine and a decrease in serotonin in the hippocampus, while it induces an increase in both in the cortex. Nicotine-induced dopamine release seems to be mediated, in part locally, by nicotinic and muscarinic receptors on dopaminergic cells. In contrast, nicotine's effect on norepinephrine and serotonin is at least partially mediated by initial changes at other than local sites, and through different receptors. Thus, the effects of nicotine and the mechanisms involved differ for different neurotransmitters and in different brain areas.