Rimonabant: an antagonist drug of the endocannabinoid system for the treatment of obesity

Obesity, an ever-increasing problem in the industrialized world, has long been a target of research for a cure or, at least, control of its expansion. In the search for treatment, the recently discovered endocannabinoid system has emerged as a new target for controlling obesity and its associated conditions. The endocannabinoid system plays an important role in controlling weight and energy balance in humans. This system is activated to a greater extent in obese patients, and the specific blockage of its receptors is the aim of rimonabant, one of the most recent drugs created for the treatment of obesity. This drug acts as a blockade for endocannabinoid receptors found in the brain and peripheral organs that play an important role on carbohydrate and fat metabolism. Clinical studies have confirmed that, when used in combination with a low calorie diet, rimonabant promotes loss in body weight, loss in abdominal circumference, and improvements in dyslipidemia. Rimonabant is also being tested as a potential anti-smoking treatment since endocannabinoids are related to the pleasurable effect of nicotine. Thus, rimonabant constitutes a new therapeutic approach to obesity and cardiovascular risk factors. Studies show effectiveness in weight loss; however, side effects such as psychiatric alterations have been reported, including depression and anxiety. These side effects have led the FDA(Food and Drug Administration) to not approve this drug in the United States. For a more complete evaluation on the safety of this drug, additional studies are in progress.     

Indirect modulation of the endocannabinoid system by specific fractions of nutmeg total extract

Context: Nutmeg [Myristica fragrans Houtt. (Myristicaceae)] has a long-standing reputation of psychoactivity. Anecdotal reports of nutmeg use as a cheap marijuana substitute, coupled to previous studies reporting a cannabimimetic-like action, suggest that nutmeg may interact with the endocannabinoid system.

Objective: The study evaluates nutmeg fractions for binding capacity with various CNS receptors and their potential interaction with the endocannabinoid system.

Materials and methods: Dichloromethane (DF) and ethyl acetate (EF) fractions were prepared from the methanol extract of powdered whole nutmeg. The HPLC-profiled fractions were assayed by the NIMH Psychoactive Drug Screening Program (PDSP) in a panel of CNS targets at a 10?μg/mL concentration. The fractions were also screened for fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL) inhibition, initially at a concentration of 500?μg/mL, then by concentration-dependent inhibition studies.

Results: None of the tested fractions showed significant binding to CNS receptors included in the PDSP panel. However, both fractions exerted significant inhibition of the FAAH and MAGL enzymes. The DF fraction inhibited FAAH and MAGL enzymes at IC₅₀ values of 21.06?±?3.16 and 15.34?±?1.61?μg/mL, respectively. Similarly, the EF fraction demonstrated FAAH and MAGL inhibition with IC₅₀ values of 15.42?±?3.09 and 11.37?±?6.15?μg/mL, respectively.

Discussion and conclusion: The study provides the first piece of evidence that nutmeg interacts with the endocannabinoid system via inhibition of the endocannabinoid catabolizing enzymes. This mechanism provides insight into reported cannabis-like action as well as expands the potential therapeutic utility of nutmeg.     

Functional neuroanatomy of the endocannabinoid system

Several components of the endocannabinoid system have been fully characterized. Among them are two types of cannabinoid receptors (termed CB1 and CB2), endogenous ligands for those receptors (referred to as "endocannabinoids"), and specific enzymes responsible for their degradation and inactivation. The study of the distribution and abundance of these elements in the central nervous system has provided the basis for the well-known effects of exogenous (both natural and synthetic) and endogenous cannabinoids. In addition, recent developments also support the idea that the endocannabinoid system plays a critical neuromodulatory role in the central nervous system. For instance, cannabinoid CB1 receptor activation is known to modify the release of several neurotransmitters, such as glutamate and gamma-aminobutyric acid. However, we still lack knowledge on fundamental aspects of the physiological roles of this system. Interestingly, changes in the distribution and activity of some of these components of the endocannabinoid system have been reported under different pathological conditions, suggesting their possible involvement in the pathogenesis of these diseases. As comprehensive excellent reviews have been recently published, the present review will focus only on the most recent advances in the field, considering a new perspective of the endocannabinoid system as composed of both neuronal and glial divisions.     

4-imidazolyl-3-amino-2-butanone (McN-A-1293), a new specific inhibitor of histidine decarboxylase

4-Imidazolyl-3-amino-2-butanone (McN-A-1293) was shown to be an effective inhibitor of fetal rat histidine decarboxylase, but not guinea pig kidney aromatic l-amino acid decarboxylase in vitro. McN-A-1293 was a more potent inhibitor than α-methylhistidine, but less potent than brocresine (NSD-1055) and other aminooxyamines. Kinetic studies using fetal rat histidine decarboxylase indicated the inhibition to be reversible and competitive with histidine. McN-A-1293 in vitro was a less potent inhibitor of diamine oxidase than brocresine and was only a weak inhibitor of imidazole-N-methyltransferase. Administration of McN-A-1293 (200 mg/kg, i.p.) to fed rats resulted in 55 per cent inhibition of gastric histidine decarboxylase, while gastric aromatic l-amino acid decarboxylase was only slightly inhibited (17 per cent). Brocresine (200 mg/kg, i.p.) inhibited both gastric decarboxylases > 60 per cent. Doses of McN-A-1293 as low as 50 mg/kg, i.p., inhibited gastric histidine decarboxylase activity. In fasted rats, McN-A-1293 (200 mg/kg, i.p.) was found to inhibit the elevation in gastric histidine decarboxylase induced by gastrin or insulin. 4-Imidazolyl-3-amino-2-butanone (McN-A-1293) is an effective and specific inhibitor of histidine decarboxylase both in vitro and in vivo and should be a useful agent for studies on the role of histidine decarboxylase in histamine physiology.     

Modulation of the endocannabinoid system by lipid rafts

Endocannabinoids like anandamide and 2-arachidonoylglycerol bind and activate type-1 (CB1R) and type-2 (CB2R) cannabinoid receptors, two inhibitory G protein-coupled receptors (GPCRs) that are localized in the central nervous system and in peripheral tissues. The biological actions of these lipids are controlled through not yet fully characterized cellular mechanisms that regulate the release of endocannabinoids from membrane precursors, their uptake by cells, and their intracellular disposal. The transport of anandamide through the plasma membrane is saturable and energy-independent, and might occur through a putative anandamide membrane transporter. Altogether anandamide and 2-arachidonoylglycerol, their congeners and the proteins that bind, transport, synthesize and hydrolyze these lipids, form the "endocannabinoid system". Accumulating evidence shows that CB1R (but not CB2R) binding and signaling, as well as anandamide transport, are under the control of lipid rafts (LRs), plasma membrane subdomains which modulate the activity of a number of GPCRs. Here we summarize the main features of the endocannabinoid system and LRs, in order to put the functional and structural effects of LRs on CB receptors, AEA transport and endocannabinoid signaling in a better focus. We outline the structural determinants that might explain the differential sensitivity of cannabic receptors towards raft integrity, and propose a general model to explain the dependence of endocannabinoid system on LRs. Finally, we also discuss the possible exploitation of LRs-targeted drugs as novel therapeutics for the treatment of endocannabinoid system-related pathologies.     

Modulation of the serotonin system by endocannabinoid signaling

The cannabinoid CB₁ receptors and their endogenous agonists, endocannabinoids (eCBs), are ubiquitously distributed throughout the central nervous system (CNS), where they play a key role in the regulation of neuronal excitability. As such, CB signaling has been implicated in the regulation of a myriad of physiological functions ranging from feeding homoeostasis to emotional and motivational processes. Ample evidence from behavioral studies also suggests that eCBs are important regulators of stress responses and a deficit in eCB signaling contributes to stress-related disorders such as anxiety and depression. The eCB-induced modulation of stress-related behaviors appears to be mediated, at least in part, through the regulation of the serotoninergic system. In this article, we review the role of eCB signaling in the regulation of the serotoninergic system with special emphasis on the cellular mechanisms by which cannabinoid CB₁ receptors modulate the excitability of dorsal raphe serotonin neurons.     

The endocannabinoid system and the regulation of the metabolism

The endocannabinoid system modulates many physiological functions by acting on receptors CB1 and CB2. The endocannabinoids are produced only when and where they are needed. They act locally and are immediately metabolised after their action. Overactivation of the endocannabinoid system is observed in obesity, with stimulation of the appetite in the hypothalamus and fat accumulation in the adipocytes with increase of insulin resistance and decrease of adiponectin. Nicotine use overactivates also the endocannabinoid system. CB1 blockade by a specific inhibitor (rimonabant) decreases food intake and weight in animal studies and increases adiponectin and insulin sensitivity. Moreover, rimonabant decreases tobacco dependence. Clinical studies with rimonabant are encouraging.     

Therapeutic potential of the endocannabinoid system in the brain

Cannabinoids have been predominantly considered as the substances responsible of the psychoactive properties of marijuana and other derivatives of Cannabis sativa. However, these compounds are now being also considered for their therapeutic potential, since the term "cannabinoid" includes much more compounds than those present in Cannabis sativa derivatives. Among them, there are numerous synthetic cannabinoids obtained by modifications from plant-derived cannabinoids, but also from the compounds that behave as endogenous ligands for the different cannabinoid receptor subtypes. Within the family of "cannabinoid-related compounds", one should also include some prototypes of selective antagonists for these receptors, and also the recently developed inhibitors of the mechanism of finalization of the biological action of endocannabinoids (transporter + FAAH). All this boom of the cannabinoid pharmacology has, therefore, an explanation in the recent discovery and characterization of the endocannabinoid signaling system, which plays a modulatory role mainly in the brain but also in the periphery. The objective of the present article will be to review, from pharmacological and biochemical points of view, the more recent advances in the study of the endocannabinoid system and their functions in the brain, as well as their alterations in a variety of pathologies and the proposed therapeutic benefits of novel cannabinoid-related compounds that improve the pharmacokinetic and pharmacodynamic properties of classic cannabinoids.     

Targeting the endocannabinoid system: future therapeutic strategies

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The endocannabinoid system and multiple sclerosis

Multiple sclerosis (MS) is a neurodegenerative disease that is characterised by repeated inflammatory/demyelinating events within the central nervous system (CNS). In addition to relapsing-remitting neurological insults, leading to loss of function, patients are often left with residual, troublesome symptoms such as spasticity and pain. These greatly diminish "quality of life" and have prompted some patients to self-medicate with and perceive benefit from cannabis. Recent advances in cannabinoid biology are beginning to support these anecdotal observations, notably the demonstration that spasticity is tonically regulated by the endogenous cannabinoid system. Recent clinical trials may indeed suggest that cannabis has some potential to relieve, pain, spasms and spasticity in MS. However, because the CB(1) cannabinoid receptor mediates both the positive and adverse effects of cannabis, therapy will invariably be associated with some unwanted, psychoactive effects. In an experimental model of MS, and in MS tissue, there are local perturbations of the endocannabinoid system in lesional areas. Stimulation of endocannabinoid activity in these areas either through increase of synthesis or inhibition of endocannabinoid degradation offers the positive therapeutic potential of the cannabinoid system whilst limiting adverse events by locally targeting the lesion. In addition, CB(1) and CB(2) cannabinoid receptor stimulation may also have anti-inflammatory and neuroprotective potential as the endocannabinoid system controls the level of neurodegeneration that occurs as a result of the inflammatory insults. Therefore cannabinoids may not only offer symptom control but may also slow the neurodegenerative disease progression that ultimately leads to the accumulation of disability.     

Targeting the endocannabinoid system in treating brain disorders

Recent cannabinoid research has a primary focus on developing therapeutics against human diseases. Many studies on cannabinoids indicate important progress for protection against several neurodegenerative disorders. Agonists of cannabinoid receptors activate signalling pathways in the brain that are linked to neuronal repair and cell maintenance, and endogenous ligands can also activate neuroprotective responses. These endocannabinoids are bioactive fatty acid amides and esters that are synthesised in the brain and include arachidonoyl ethanolamide (anandamide) and 2-arachidonoyl glycerol. Endocannabinoids are released in response to pathogenic events, thus representing a potential compensatory repair mechanism. Enhancing this on-demand action of endocannabinoids is a strategy with which to promote endogenous repair signalling. For such enhancement, considerable work has gone into modulating the availability of endocannabinoids by blocking the processes of their deactivation. The targets include the anandamide-hydrolysing enzyme fatty acid amide hydrolase, the carrier-mediated anandamide transport system and 2-arachidonoyl glycerol-deactivating enzyme monoacylglycerol lipase. The activity of endocannabinoids is terminated through transport and degradation and, accordingly, selective inhibitors of these processes effectively exploit the protective nature of cannabinergic responses. This review highlights recent studies implicating the endocannabinoid system in neuroprotection against different disorders of the CNS.     

Targeting the endocannabinoid system in treating brain disorders

Recent cannabinoid research has a primary focus on developing therapeutics against human diseases. Many studies on cannabinoids indicate important progress for protection against several neurodegenerative disorders. Agonists of cannabinoid receptors activate signalling pathways in the brain that are linked to neuronal repair and cell maintenance, and endogenous ligands can also activate neuroprotective responses. These endocannabinoids are bioactive fatty acid amides and esters that are synthesised in the brain and include arachidonoyl ethanolamide (anandamide) and 2-arachidonoyl glycerol. Endocannabinoids are released in response to pathogenic events, thus representing a potential compensatory repair mechanism. Enhancing this on-demand action of endocannabinoids is a strategy with which to promote endogenous repair signalling. For such enhancement, considerable work has gone into modulating the availability of endocannabinoids by blocking the processes of their deactivation. The targets include the anandamide-hydrolysing enzyme fatty acid amide hydrolase, the carrier-mediated anandamide transport system and 2-arachidonoyl glycerol-deactivating enzyme monoacylglycerol lipase. The activity of endocannabinoids is terminated through transport and degradation and, accordingly, selective inhibitors of these processes effectively exploit the protective nature of cannabinergic responses. This review highlights recent studies implicating the endocannabinoid system in neuroprotection against different disorders of the CNS.     

The endocannabinoid system and Huntington's disease

The research in Huntington's disease (HD) has been growing exponentially during the last decade, since the discovery of the genetic basis that leads to neurodegeneration. HD is one of several progressive neurodegenerative disorders, in which the underlying mutation is a CAG expansion encoding a polyglutamine tract in a specific protein, which in the case of HD, is called huntingtin. The first clinical symptoms of HD are generally psychiatric abnormalities, most commonly depression and mood disturbances. Involuntary choreiform movements and dementia develop over the next 15-20 years, and death generally results from complications derived from immobility. There is currently no cure, or even an effective therapy to offset the decline in mental and motor capabilities suffered by those affected by HD, but recent studies have started to examine the usefulness of different classes of new compounds. Among these, plant-derived, synthetic or endogenous cannabinoids have been proposed to have therapeutic value for the treatment of HD, since they act on cannabinoid CB(1) receptors located in the basal ganglia circuitry, that is affected by the striatal atrophy typical of HD. Recent studies have characterized the changes in these receptors, as well as their endogenous ligands, in the basal ganglia in a variety of animal models of HD. The results are indicative that the endocannabinoid system becomes hypofunctional in this disease, which could be related to the hyperkinesia typical of the earliest phases of this disease. In addition, it has been proposed that the loss of these receptors might be involved in the process of pathogenesis itself. This, together with the well-known protective properties of cannabinoid-related compounds, suggest that, in addition to a symptomatic usefulness, cannabinoids might also serve to delay or to arrest the development of this disease. The present article will review all recent data dealing with the biochemical, pharmacological and therapeutic bases that support a potential role of cannabinoids in the pathogenesis and/or therapeutic treatment of this motor disorder.     

Manipulation of the endocannabinoid system by a general anaesthetic

An article appearing in this issue of the British Journal of Pharmacology shows for the first time that the general anaesthetic propofol inhibits one of the enzymes catalysing endocannabinoid hydrolysis and inactivation, the fatty acid amide hydrolase, thereby enhancing the brain levels of anandamide and 2-arachidonoylglycerol in mouse brain. The authors provide evidence that this effect of propofol underlies part of the sedative effects of this compound. The importance of these findings in the light of the likely role of the endocannabinoid system in the control of sleep-wake cycles, and of the possibility of developing therapeutic drugs from substances that manipulate endocannabinoid levels, is discussed.     

Role of the endocannabinoid system in depression and suicide

Depression is one of the most prevalent forms of neuropsychiatric disorder and is a major cause of suicide worldwide. The prefrontal cortex is a crucial brain region that is thought to be involved in the regulation of mood, aggression and/or impulsivity and decision making, which are altered in suicidality. Evidence of the role of the endocannabinoid (EC) system in the neurobiology of neuropsychiatric disorders is beginning to emerge. The behavioral effects of ECs are believed to be mediated through the central cannabinoid CB1 receptor. Alterations in the levels of ECs, and in the density and coupling efficacy of CB1 receptors, have been reported in the prefrontal cortex of depressed and alcoholic suicide victims. These findings support our hypothesis that altered EC function contributes to the pathophysiological aspects of suicidal behavior. Here, we provide a brief overview of the role of the EC system in alcoholism, depression and suicide, and discuss possible therapeutic interventions and directions for future research.     

Characterisation of the endocannabinoid system in rat haemochorial placenta

Trophoblast cells that comprise the placenta play a crucial role in the complex cross-talk between fetus and maternal tissues. Although anandamide and 2-arachidonoylglycerol, the best studied endocannabinoids, affect trophoblast attachment and outgrowth, the functional significance of the endocannabinoid system in the development of placenta has not been established. We investigated the correlation between endocannabinoid levels and the pattern of expression of the receptors and metabolic enzymes of the endocannabinoid system during rat placental development. Here, we showed that all the endocannabinoid machinery is dynamically expressed in the functionally distinct basal and labyrinth zones of the rat placenta. Indeed, endocannabinoid levels are shown to increase with the progression of pregnancy. Together, these data support a role for the endocannabinoid system in normal placental function and evidence for a potential novel cellular target for the deleterious action of cannabis-derived compounds during the second half of pregnancy.