Blockade by the cannabinoid CB1 receptor antagonist, rimonabant (SR141716), of the potentiation by quinelorane of food-primed reinstatement of food-seeking behavior

Binge eating, overeating and food addiction: Approaches for examining food overconsumption in laboratory rodents

Overeating ranges in severity from casual overindulgence to an overwhelming drive to consume certain foods. At its most extreme, overeating can manifest as clinical diagnoses such as binge eating disorder or bulimia nervosa, yet subclinical forms of overeating such as emotional eating or uncontrolled eating can still have a profoundly negative impact on health and wellbeing. Although rodent models cannot possibly capture the full spectrum of disordered overeating, studies in laboratory rodents have substantially progressed our understanding of the neurobiology of overconsumption. These experimental approaches range from simple food-exposure protocols that promote binge-like eating and the development of obesity, to more complex operant procedures designed to examine distinct 'addiction-like' endophenotypes for food. This review provides an overview of these experimental approaches, with the view to providing a comprehensive resource for preclinical investigators seeking to utilize behavioural models for studying the neural systems involved in food overconsumption.

Pep19 Has a Positive Effect on Insulin Sensitivity and Ameliorates Both Hepatic and Adipose Tissue Phenotype of Diet-Induced Obese Mice

Peptide DIIADDEPLT (Pep19) has been previously suggested to improve metabolic parameters, without adverse central nervous system effects, in a murine model of diet-induced obesity. Here, we aimed to further evaluate whether Pep19 oral administration has anti-obesogenic effects, in a well-established high-fat diet-induced obesity model. Male Swiss mice, fed either a standard diet (SD) or high-fat diet (HFD), were orally administrated for 30 consecutive days, once a day, with saline vehicle or Pep19 (1 mg/kg). Next, several metabolic, morphological, and behavioral parameters were evaluated. Oral administration of Pep19 attenuated HFD body-weight gain, reduced in approximately 40% the absolute mass of the endocrine pancreas, and improved the relationship between circulating insulin and peripheral insulin sensitivity. Pep19 treatment of HFD-fed mice attenuated liver inflammation, hepatic fat distribution and accumulation, and lowered plasma alanine aminotransferase activity. The inguinal fat depot from the SD group treated with Pep19 showed multilocular brown-fat-like cells and increased mRNA expression of uncoupling protein 1 (UCP1), suggesting browning on inguinal white adipose cells. Morphological analysis of brown adipose tissue (BAT) from HFD mice showed the presence of larger white-like unilocular cells, compared to BAT from SD, Pep19-treated SD or HFD mice. Pep19 treatment produced no alterations in mice behavior. Oral administration of Pep19 ameliorates some metabolic traits altered by diet-induced obesity in a Swiss mice model.

A behavioral and pharmacological characterization of palatable diet alternation in mice

Obesity and eating disorders are widespread in Western societies. Both the increased availability of highly palatable foods and dieting are major risk factors contributing to the epidemic of disorders of feeding. The purpose of this study was to characterize an animal model of maladaptive feeding induced by intermittent access to a palatable diet alternation in mice. In this study, mice were either continuously provided with standard chow food (Chow/Chow), or provided with standard chow for 2days and a high-sucrose, palatable food for 1day (Chow/Palatable). Following stability of intake within the cycling paradigm, we then investigated the effects of several pharmacological treatments on excessive eating of palatable food: naltrexone, an opioid receptor antagonist, SR141716A, a cannabinoid-1 receptor antagonist/inverse agonist, and BD-1063, a sigma-1 receptor antagonist. Over successive cycles, Chow/Palatable mice showed an escalation of palatable food intake within the first hour of renewed access to palatable diet and displayed hypophagia upon its removal. Naltrexone, SR141716A, and BD-1063 all reduced overconsumption of palatable food during this first hour. Here we provide evidence of strong face and convergent validity in a palatable diet alternation model in mice, confirming multiple shared underlying mechanisms of pathological eating across species, and thus making it a useful therapeutic development tool.

G protein coupled receptor 18: A potential role for endocannabinoid signaling in metabolic dysfunction

Endocannabinoids are products of dietary fatty acids that are modulated by an alteration in food intake levels. Overweight and obese individuals have substantially higher circulating levels of the arachidonic acid derived endocannabinoids, anandamide and 2-arachidonoyl glycerol, and show an altered pattern of cannabinoid receptor expression. These cannabinoid receptors are part of a large family of G protein coupled receptors (GPCRs). GPCRs are major therapeutic targets for various diseases within the cardiovascular, neurological, gastrointestinal, and endocrine systems, as well as metabolic disorders such as obesity and type 2 diabetes mellitus. Obesity is considered a state of chronic low-grade inflammation elicited by an immunological response. Interestingly, the newly deorphanized GPCR (GPR18), which is considered to be a putative cannabinoid receptor, is proposed to have an immunological function. In this review, the current scientific knowledge on GPR18 is explored including its localization, signaling pathways, and pharmacology. Importantly, the involvement of nutritional factors and potential dietary regulation of GPR18 and its (patho)physiological roles are described. Further research on this receptor and its regulation will enable a better understanding of the complex mechanisms of GPR18 and its potential as a novel therapeutic target for treating metabolic disorders.

Individual differences in response to positive and negative stimuli: endocannabinoid-based insight on approach and avoidance behaviors

Approach and avoidance behaviors-the primary responses to the environmental stimuli of danger, novelty and reward-are associated with the brain structures that mediate cognitive functionality, reward sensitivity and emotional expression. Individual differences in approach and avoidance behaviors are modulated by the functioning of amygdaloid-hypothalamic-striatal and striatal-cerebellar networks implicated in action and reaction to salient stimuli. The nodes of these networks are strongly interconnected and by acting on them the endocannabinoid and dopaminergic systems increase the intensity of appetitive or defensive motivation. This review analyzes the approach and avoidance behaviors in humans and rodents, addresses neurobiological and neurochemical aspects of these behaviors, and proposes a possible synaptic plasticity mechanism, related to endocannabinoid-dependent long-term potentiation (LTP) and depression that allows responding to salient positive and negative stimuli.

Behavioral and electrophysiological effects of endocannabinoid and dopaminergic systems on salient stimuli

Rewarding effects have been related to enhanced dopamine (DA) release in corticolimbic and basal ganglia structures. The DAergic and endocannabinoid interaction in the responses to reward is described. This study investigated the link between endocannabinoid and DAergic transmission in the processes that are related to response to two types of reward, palatable food and novelty. Mice treated with drugs acting on endocannabinoid system (ECS) (URB597, AM251) or DAergic system (haloperidol) were submitted to approach-avoidance conflict tasks with palatable food or novelty. In the same mice, the cannabinoid type-1 (CB₁)-mediated GABAergic transmission in medium spiny neurons of the dorsomedial striatum was analyzed. The endocannabinoid potentiation by URB597 magnified approach behavior for reward (food and novelty) and in parallel inhibited dorsostriatal GABAergic neurotransmission. The decreased activity of CB₁ receptor by AM251 (alone or with URB597) or of DAergic D₂ receptor by haloperidol had inhibitory effects toward the reward and did not permit the inhibition of dorsostriatal GABAergic transmission. When haloperidol was coadministered with URB597, a restoration effect on reward and reward-dependent motor activity was observed, only if the reward was the palatable food. In parallel, the coadministration led to restoring inhibition of CB₁-mediated GABAergic transmission. Thus, in the presence of simultaneous ECS activation and inhibition of DAergic system the response to reward appears to be a stimulus-dependent manner.

Moderation of antipsychotic-induced weight gain by energy balance gene variants in the RUPP autism network risperidone studies

Second-generation antipsychotic exposure, in both children and adults, carries significant risk for excessive weight gain that varies widely across individuals. We queried common variation in key energy balance genes (FTO, MC4R, LEP, CNR1, FAAH) for their association with weight gain during the initial 8 weeks in the two NIMH Research Units on Pediatric Psychopharmacology Autism Network trials (N=225) of risperidone for treatment of irritability in children/adolescents aged 4-17 years with autism spectrum disorders. Variants in the cannabinoid receptor (CNR)-1 promoter (P=1.0 × 10(-6)), CNR1 (P=9.6 × 10(-5)) and the leptin (LEP) promoter (P=1.4 × 10(-4)) conferred robust-independent risks for weight gain. A model combining these three variants was highly significant (P=1.3 × 10(-9)) with a 0.85 effect size between lowest and highest risk groups. All results survived correction for multiple testing and were not dependent on dose, plasma level or ethnicity. We found no evidence for association with a reported functional variant in the endocannabinoid metabolic enzyme, fatty acid amide hydrolase, whereas body mass index-associated single-nucleotide polymorphisms in FTO and MC4R showed only trend associations. These data suggest a substantial genetic contribution of common variants in energy balance regulatory genes to individual antipsychotic-associated weight gain in children and adolescents, which supersedes findings from prior adult studies. The effects are robust enough to be detected after only 8 weeks and are more prominent in this largely treatment naive population. This study highlights compelling directions for further exploration of the pharmacogenetic basis of this concerning multifactorial adverse event.

Rationale and consequences of reclassifying obesity as an addictive disorder: neurobiology, food environment and social policy perspectives

The rapid increase in the prevalence of obesity is a priority for investigators from across numerous disciplines, including biology, nutritional science, and public health and policy. In this paper, we systematically examine the premise that common dietary obesity is an addictive disorder, based on the criteria for addiction described in the Diagnostic and Statistical Manual (DSM) of Mental Disorders of the American Psychiatric Association, version IV, and consider the consequences of such a reclassification of obesity for public policy. Specifically, we discuss evidence from both human and animal studies investigating the effects of various types and amounts of food and the food environment in obese individuals. Neurobiological studies have shown that the hedonic brain pathways activated by palatable food overlap considerably with those activated by drugs of abuse and suffer significant deficits after chronic exposure to high-energy diets. Furthermore, food as a stimulus can induce the sensitization, compulsion and relapse patterns observed in individuals who are addicted to illicit drugs. The current food environment encourages these addictive-like behaviors where increased exposure through advertisements, proximity and increased portion sizes are routine. Taking lessons from the tobacco experience, it is clear that reclassifying common dietary obesity as an addictive disorder would necessitate policy changes (e.g., regulatory efforts, economic strategies, and educational approaches). These policies could be instrumental in addressing the obesity epidemic, by encouraging the food industry and the political leadership to collaborate with the scientific and medical community in establishing new and more effective therapeutic approaches.

Sensitization to cocaine is inhibited after intra-accumbal GR103691 or rimonabant, but it is enhanced after co-infusion indicating functional interaction between accumbens D(3) and CB1 receptors

RATIONALE

Dopamine D(3) receptors and cannabinoid CB(1) receptors are both expressed in the nucleus accumbens, and they have been involved in motor sensitization to cocaine. The objectives were: (1) to study the effects of blockade of these receptors on sensitization to repeated cocaine, by using GR103691, D(3) receptor blocker, and rimonabant, CB(1) receptor ligand, and (2) to discern if both receptors interact by co-infusing them.

MATERIALS AND METHODS

Cocaine (10 mg/kg) was injected daily for 3 days (induction phase) and later on day 8 (expression phase), and locomotor activity was measured during 2 h after cocaine. GR103691 and rimonabant were bilaterally injected (0.5 μl volume of each infusion) in the nucleus accumbens through cannulae (GR103691, 0, 4.85, and 9.7 μg/μl; rimonabant, 0, 0.5, and 1.5 μg/μl), before cocaine, during either induction or expression phases of sensitization.

RESULTS

The findings indicated that sensitizing effects of cocaine were abolished after D(3) receptor blocking during both induction and expression phases, as well as rimonabant infusion during the expression (not induction) phase. A functional interaction between both receptors was also observed, because if GR103691 was injected during induction and rimonabant during expression, sensitizing effects of cocaine were observed to be normal or further enhanced.

CONCLUSION

Dopamine D(3) receptors within the nucleus accumbens are critical for the development and consolidation of sensitization, and cannabinoid CB(1) receptors are critical for the expression of sensitization. Co-blockade of D(3) and CB(1) receptors exert opposite effects to blockade of these receptors separately, revealing the existence of a functional interaction between them.

Cannabinoid-1 receptor gene deletion has a compartment-specific affect on the dendritic and axonal availability of μ-opioid receptors and on dopamine axons in the mouse nucleus accumbens

Cannabinoid-type 1 (CB1) receptors are implicated in μ-opioid receptor (μ-OR)-dependent reward ascribed partially to mesolimbic dopamine release in the nucleus accumbens (Acb) shell. Thus, CB1 receptor gene deletion may preferentially alter the availability of μ-ORs and/or dopamine innervation in this brain region, which is functionally distinct from the motor-associated Acb core. To test this hypothesis, we examined the electron microscopic immunolabeling of the μ-OR and the dopamine-synthesizing enzyme, tyrosine hydroxylase (TH) in Acb shell, and core of adult C57BL/6J wild-type (WT) and CB1-knock-out (KO) mice. The μ-OR-immunogold particles were observed in the cytoplasm and on the plasmalemma in dendrites, dendritic spines, and axon terminals throughout the Acb. Compared to WT, the Acb shell of CB1-KO mice showed a lower cytoplasmic density of μ-ORs in dendrites and fewer μ-OR labeled, but not unlabeled, dendritic spines. In this region, the CB1-KO's had a significantly enhanced plasmalemmal density of μ-OR-immunogold in axon terminals, 70% of which formed excitatory-type synapses. However, the number of both μ-OR-labeled terminals and TH-labeled small varicosities was significantly reduced in the Acb shell of CB1-KO's. These adaptations were not seen in the Acb core, where CB1-KO's had a preferentially lower dendritic plasmalemmal and total spine density of μ-OR immunogold. Our results indicate that constitutive deletion of the CB1 receptor gene has a major impact on the pre and postsynaptic availability of μ-ORs at axospinous synapses and on the dopamine innervation of the Acb shell as well as the dendritic surface expression of μ-ORs in Acb core of mature rodents.

The neuropharmacology of relapse to food seeking: methodology, main findings, and comparison with relapse to drug seeking

Relapse to old, unhealthy eating habits is a major problem in human dietary treatments. The mechanisms underlying this relapse are unknown. Surprisingly, until recently this clinical problem has not been systematically studied in animal models. Here, we review results from recent studies in which a reinstatement model (commonly used to study relapse to abused drugs) was employed to characterize the effect of pharmacological agents on relapse to food seeking induced by either food priming (non-contingent exposure to small amounts of food), cues previously associated with food, or injections of the pharmacological stressor yohimbine. We also address methodological issues related to the use of the reinstatement model to study relapse to food seeking, similarities and differences in mechanisms underlying reinstatement of food seeking versus drug seeking, and the degree to which the reinstatement procedure provides a suitable model for studying relapse in humans. We conclude by discussing implications for medication development and future research. We offer three tentative conclusions: (1)The neuronal mechanisms of food-priming- and cue-induced reinstatement are likely different from those of reinstatement induced by the pharmacological stressor yohimbine. (2)The neuronal mechanisms of reinstatement of food seeking are possibly different from those of ongoing food-reinforced operant responding. (3)The neuronal mechanisms underlying reinstatement of food seeking overlap to some degree with those of reinstatement of drug seeking.

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.

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.

Cannabinoids in eating disorders and obesity

Cannabinoid system is a crucial mechanism in regulating food intake and energy metabolism. It is involved in central and peripheral mechanisms regulating such behavior, interacting with many other signaling systems with a role in metabolic regulation. Cannabinoid agonists promote food intake, and soon a cannabinoid antagonist, rimonabant, will be marketed for the treatment of obesity. It not only causes weight loss, but also alleviates metabolic syndrome. We present a review of current knowledge on this subject, along with data from our own research: genetic studies on this system in eating disorders and obesity and studies locating cannabinoid receptors in areas related to food intake. Such studies suggest cannabinoid hyperactivity in obesity, and this excessive activity may have prognostic implications.

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.

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.

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.

The structural evolution of dopamine D3 receptor ligands: structure-activity relationships and selected neuropharmacological aspects

"Evolution consists largely of molecular tinkering."-Following the famous concept of the molecular geneticist and medicine Nobel laureate François Jacob, in this review we describe the structural evolution of dopamine D3 receptor ligands from the natural agonist dopamine (DA) to highly potent and subtype selective new agents by bioisosteric tinkering with well-established and privileged or novel and fancy chemical functionalities and scaffolds. Some of the more than 200 ligands presented herein have already achieved therapeutic or scientific value up to now, some will most likely achieve it in the future. Hence, great importance is not only attached to the relationship between structure and activity of the ligands, but also to their utility as pharmacological tools in animal models or as therapeutics in patients with neurological diseases or other disorders.

Targeting dopamine D2 and cannabinoid-1 (CB1) receptors in rat nucleus accumbens

The nucleus accumbens (Acb) shell and core are essential components of neural circuitry mediating the reward and motor effects produced by activation of dopamine D2 or cannabinoid-1 (CB1) receptors. D2 receptors can form heterodimeric complexes with cannabinoid-1 (CB1) receptors and are also involved in control of the availability of both dopamine and endocannabinoids. Thus, the subcellular locations of D2 and CB1 receptors with respect to each other are implicit to their physiological actions in the Acb. We used electron microscopic immunocytochemistry to determine these locations in the Acb shell and core of rat brain. In each region, many neuronal profiles showed endomembrane and plasmalemmal distributions of one or both receptors. Approximately one-third of the labeled profiles were somata and dendrites, some of which showed overlapping subcellular distributions of D2 and CB1 immunoreactivities. The remaining labeled profiles were small axons and axon terminals containing CB1 and/or D2 receptors. Of the labeled terminals forming recognizable synapses, approximately 20% of those containing CB1 receptors contacted D2-labeled dendrites, while conversely, almost 15% of those containing D2 receptors contacted CB1-labeled dendrites. These results provide the first ultrastructural evidence that D2 and CB1 receptors in the Acb shell and core have subcellular distributions supporting both intracellular associations and local involvement of D2 receptors in making available endocannabinoids that are active on CB1 receptors in synaptic neurons. These distributions have direct relevance to the rewarding and euphoric as well as motor effects produced by marijuana and by addictive drugs enhancing dopamine levels in the Acb.

Endocannabinoids in the regulation of appetite and body weight

The discovery of cannabinoid receptors, together with the development of selective cannabinoid receptor antagonists, has encouraged a resurgence of cannabinoid pharmacology. With the identification of endogenous agonists, such as anandamide, scientists have sought to uncover the biological role of endocannabinoid systems; initially guided by the long-established actions of cannabis and exogenous cannabinoids such as delta9-tetrahydrocannabinol (THC). In particular, considerable research has examined endocannabinoid involvement in appetite, eating behaviour and body weight regulation. It is now confirmed that endocannabinoids, acting at brain CB1 cannabinoid receptors, stimulate appetite and ingestive behaviours, partly through interactions with more established orexigenic and anorexigenic signals. Key structures such as the nucleus accumbens and hypothalamic nuclei are sensitive sites for the hyperphagic actions of these substances, and endocannabinoid activity in these regions varies in relation to nutritional status and feeding expression. Behavioural studies indicate that endocannabinoids increase eating motivation by enhancing the incentive salience and hedonic evaluation of ingesta. Moreover, there is strong evidence of an endocannabinoid role in energy metabolism and fuel storage. Recent developments point to potential clinical benefits of cannabinoid receptor antagonists in the management of obesity, and of agonists in the treatment of other disorders of eating and body weight regulation.

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.

Endocannabinoid control of food intake and energy balance

Marijuana and its major psychotropic component, Delta(9)-tetrahydrocannabinol, stimulate appetite and increase body weight in wasting syndromes, suggesting that the CB(1) cannabinoid receptor and its endogenous ligands, the endocannabinoids, are involved in controlling energy balance. The endocannabinoid system controls food intake via both central and peripheral mechanisms, and it may also stimulate lipogenesis and fat accumulation. Here we discuss the multifaceted regulation of energy homeostasis by endocannabinoids, together with its applications to the treatment of eating disorders and metabolic syndromes.

Control of the reinforcing effects of nicotine by associated environmental stimuli in animals and humans

Tobacco dependence through cigarette smoking is the leading preventable cause of death in the world and kills nearly 4 million people annually. Nicotine, a psychoactive component of tobacco, is thought to have a major role in tobacco dependence by acting directly as a reinforcer of drug-seeking and drug-taking behavior. However, recent findings obtained with two procedures that are used widely to assess reinforcing effects of drugs in experimental animals, intravenous drug self-administration and conditioned place-preference procedures, demonstrate that environmental factors have a major influence on the reinforcing effects of nicotine. Under some experimental conditions, nicotine is also self-administered reliably by humans. Environmental stimuli that have been associated previously with the self-administration of nicotine can reinstate extinguished drug-seeking behavior in animals and precipitate relapse to smoking behavior in ex-smokers. Innovative medications that target cannabinoid CB(1) and dopamine D(3) receptors and might block specifically the influence of such conditioned environmental stimuli in smokers are in development.

Inhibition of restraint stress-induced neural and behavioural activation by endogenous cannabinoid signalling

The role of endocannabinoid (eCB) signalling in restraint stress-induced neuronal activation was studied. Male mice exposed to 30 min of restraint exhibit increased Fos protein within prefrontal cortex (PFC), lateral septum (LS), nucleus accumbens (Acb) and medial amygdala. SR141716 (2 mg/kg) itself had no effect on Fos but pretreatment with SR141716 significantly potentiated restraint-induced Fos expression in cingulate, LS and Acb. SR141716 also significantly increased the time spent in active escape behaviours during the restraint. In restraint-habituated mice (mice exposed to four previous restraint episodes), the fifth restraint exposure resulted in decreased expression of active escape behaviours compared to the first exposure and only induced Fos protein in the central and medial amygdala. Administration of SR141716 prior to the fifth restraint episode resulted in greater potentiation of restraint-induced Fos induction than the first; significant increases occurred within all regions of PFC examined, LS and Acb. Brain regional eCB content was measured immediately after restraint. N-arachidonylethanolamine content within the amygdala was significantly decreased after both restraint episodes. 2-Arachidonylglycerol content was significantly increased in both the limbic forebrain and amygdala after the fifth restraint but not the first. Restraint had no effect on cerebellar eCB content. These data suggest that eCB activation of CB(1) receptors opposes the behavioural and neuronal responses to aversive stimuli. Because repeated homotypic stress increased both limbic 2-AG and resulted in a greater effect of SR141716 on limbic Fos expression, we hypothesize that increased CB(1) receptor activity contributes to the expression of habituation to homotypic stress.

Synergistic interactions between cannabinoids and environmental stress in the activation of the central amygdala

Anxiety and panic are the most common adverse effects of cannabis intoxication; reactions potentiated by stress. Data suggest that cannabinoid (CB1) receptor modulation of amygdalar activity contributes to these phenomena. Using Fos as a marker, we tested the hypothesis that environmental stress and CB1 cannabinoid receptor activity interact in the regulation of amygdalar activation in male mice. Both 30 min of restraint and CB1 receptor agonist treatment (Delta9-tetrahydrocannabinol (2.5 mg/kg) or CP55940 (0.3 mg/kg); by i.p. injection) produced barely detectable increases in Fos expression within the central amygdala (CeA). However, the combination of restraint and CB1 agonist administration produced robust Fos induction within the CeA, indicating a synergistic interaction between environmental stress and CB1 receptor activation. An inhibitor of endocannabinoid transport, AM404 (10 mg/kg), produced an additive interaction with restraint within the CeA. In contrast, fatty acid amide hydrolase (FAAH) inhibitor-treated mice (URB597, 1 mg/kg) and FAAH-/- mice did not exhibit any differences in amygdalar activation in response to restraint compared to control mice. In the basolateral (BLA) and medial amygdala, restraint stress produced a low level of Fos induction, which was unaffected by cannabinoid treatment. Interestingly, the CB1 receptor antagonist SR141716 dose-dependently increased Fos expression in the BLA and CeA. These data suggest the CeA is an important neural substrate subserving the interactions between cannabinoids and environmental stress, and could be relevant to understanding the context-dependent emotional and affective changes induced by marijuana intoxication and the role of endocannabinoid signaling in the modulation of amygdalar activity.

Cannabinoid CB1 receptor antagonists as promising new medications for drug dependence

This review examines the development of cannabinoid CB(1) receptor antagonists as a new class of therapeutic agents for drug addiction. Abused drugs [alcohol, opiates, Delta(9)-tetrahydrocannabinol (Delta(9)-THC), and psychostimulants, including nicotine] elicit a variety of chronically relapsing disorders by interacting with endogenous neural pathways in the brain. In particular, they share the common property of activating mesolimbic dopamine brain reward systems, and virtually all abused drugs elevate dopamine levels in the nucleus accumbens. Cannabinoid CB(1) receptors are expressed in this brain reward circuit and modulate the dopamine-releasing effects of Delta(9)-THC and nicotine. Rimonabant (SR141716), a CB(1) receptor antagonist, blocks both the dopamine-releasing and discriminative and rewarding effects of Delta(9)-THC in animals. Blockade of CB(1) receptor activity by genetic invalidation also decreases rewarding effects of opiates and alcohol in animals. Although CB(1) receptor blockade is generally ineffective in reducing the self-administration of cocaine in rodents and primates, it reduces the reinstatement of extinguished cocaine-seeking behavior produced by cocaine-associated conditioned stimuli and cocaine-priming injections. Likewise, CB(1) receptor blockade is effective in reducing nicotine-seeking behavior induced by re-exposure to nicotine-associated stimuli. Some of these findings have been recently validated in humans. In clinical trials, Rimonabant blocks the subjective effects of Delta(9)-THC in humans and prevents relapse to smoking in exsmokers. Findings from both clinical and preclinical studies suggest that ligands blocking CB(1) receptors offer a novel approach for patients suffering from drug dependence that may be efficacious across different classes of abused drugs.

Phenotypic analysis of dopamine receptor knockout mice; recent insights into the functional specificity of dopamine receptor subtypes

The functional specificity of dopamine receptor subtypes remains incompletely understood, in part due to the absence of highly selective agonists and antagonists. Phenotypic analysis of dopamine receptor knockout mice has been instrumental in identifying the role of dopamine receptor subtypes in mediating dopamine's effects on motor function, cognition, reward, and emotional behaviors. In this article, we provide an update of recent studies in dopamine receptor knockout mice and discuss the limitations and future promise of this approach.

Cannabinoid physiology and pharmacology: 30 years of progress

Delta9-Tetrahydrocannabinol from Cannabis sativa is mimicked by cannabimimetic analogs such as CP55940 and WIN55212-2, and antagonized by rimonabant and SR144528, through G-protein-coupled receptors, CB1 in the brain, and CB2 in the immune system. Eicosanoids anandamide and 2-arachidonoylglycerol are the "endocannabinoid" agonists for these receptors. CB1 receptors are abundant in basal ganglia, hippocampus and cerebellum, and their functional activity can be mapped during behaviors using cerebral metabolism as the neuroimaging tool. CB1 receptors couple to G(i/o) to inhibit cAMP production, decrease Ca2+ conductance, increase K+ conductance, and increase mitogen-activated protein kinase activity. Functional activation of G-proteins can be imaged by [35S]GTPgammaS autoradiography. Post-synaptically generated endocannabinoids form the basis of a retrograde signaling mechanism referred to as depolarization-induced suppression of inhibition (DSI) or excitation (DSE). Under circumstances of sufficient intracellular Ca2+ (e.g., burst activity in seizures), synthesis of endocannabinoids releases a diffusible retrograde messenger to stimulate presynaptic CB1 receptors. This results in suppression of gamma-aminobutyric acid (GABA) release, thereby relieving the post-synaptic inhibition. Tolerance develops as neurons adjust both receptor number and cellular signal transduction to the chronic administration of cannabinoid drugs. Future therapeutic drug design can progress based upon our current understanding of the physiology and pharmacology of CB1, CB2 and related receptors. One very important role for CB1 antagonists will be in the treatment of craving in the disease of substance abuse.

Rimonabant, a CB1 antagonist, blocks nicotine-conditioned place preferences

The effects of Rimonabant (SR141716), an antagonist at cannabinoid CB1 receptors, were evaluated in animal models for subjective and rewarding effects of nicotine. Acute administration of 1 or 3 mg/kg SR141716 blocked expression of nicotine-induced conditioned place preferences. SR141716 (0.3-3 mg/kg) was also studied in rats trained to discriminate nicotine from saline under a fixed-ratio schedule of food delivery. In contrast to nicotine replacement therapy and bupropion, SR141716 did not have nicotine-like discriminative effects and did not alter the dose-response curve for nicotine discrimination. These findings support the proposed use of SR141716 for smoking cessation and indicate that it would selectively reduce the influence of environmental stimuli that contribute to persistent smoking behavior, without affecting subjective responses to nicotine.

CB1 receptor agonist and heroin, but not cocaine, reinstate cannabinoid-seeking behaviour in the rat

We recently provided evidence for a functional link between cannabinoid and opioid endogenous systems in relapse to heroin-seeking behaviour in rats. In the present study, we aimed at investigating whether the previously observed cross-talk between cannabinoids and opioids could be extended to mechanisms underlying relapse to cannabinoid-seeking behaviour after a prolonged period of abstinence. In rats previously trained to intravenously self-administer the synthetic cannabinoid receptor (CB1) agonist WIN 55,212-2 (12.5 microg kg(-1) inf(-1)) under a fixed ratio (FR1) schedule of reinforcement, noncontingent nonreinforced intraperitoneal (i.p.) priming injections of the previously self-administered CB1 agonist (0.25 and 0.5 mg kg(-1)) as well as heroin (0.5 mg kg(-1)), but not cocaine (10 mg kg(-1)), effectively reinstate cannabinoid-seeking behaviour following 3 weeks of extinction. The selective CB1 receptor antagonist SR 141716A (0.3 mg kg(-1) i.p.) does not reinstate responding when given alone, but completely prevents the cannabinoid-seeking behaviour triggered by WIN 55,212-2 or heroin primings. The nonselective opioid antagonist naloxone (1 mg kg(-1) i.p.) has no effect on operant behaviour per se, but significantly blocks cannabinoid- and heroin-induced reinstatement of cannabinoid-seeking behaviour. These results provide the first evidence of drug-induced reinstatement of cannabinoid-seeking behaviour, and further strengthen previous findings on a cross-talk between the endogenous cannabinoid and opioid systems in relapse mechanisms to drug-seeking.