CB(1) signaling in forebrain and sympathetic neurons is a key determinant of endocannabinoid actions on energy balance

Central control of thermogenesis

In mammals and birds, conservation of body heat at around 37 °C is vital to life. Thermogenesis is the production of this heat which can be obligatory, as in basal metabolic rate, or it can be facultative such as the response to cold. A complex regulatory system has evolved which senses environmental or core temperature and integrates this information in hypothalamic regions such as the preoptic area and dorsomedial hypothalamus. These areas then send the appropriate signals to generate and conserve heat (or dissipate it). In this review, the importance of the sympathetic nervous system is discussed in relation to its role in basal metabolic rate and adaptive thermogenesis with a particular emphasis to human obesity. The efferent sympathetic pathway does not uniformly act on all tissues; different tissues can receive different levels of sympathetic drive at the same time. This is an important concept in the discussion of the pharmacotherapy of obesity. Despite decades of work the medicine chest contains only one pill for the long term treatment of obesity, orlistat, a lipase inhibitor that prevents the absorption of lipid from the gut and is itself not systemically absorbed. The central controlling system for thermogenesis has many potential intervention points. Several drugs, previously marketed, awaiting approval or in the earlier stages of development may have a thermogenic effect via activation of the sympathetic nervous system at some point in the thermoregulatory circuit and are discussed in this review. If the balance is weighted to the "wrong" side there is the burden of increased cardiovascular risk while a shift to the "right" side, if possible, will afford a thermogenic benefit that is conducive to weight loss maintenance. This article is part of a Special Issue entitled 'Central Control Food Intake'

The central cannabinoid CB1 receptor is required for diet-induced obesity and rimonabant's antiobesity effects in mice

Cannabinoid receptor CB1 is expressed abundantly in the brain and presumably in the peripheral tissues responsible for energy metabolism. It is unclear if the antiobesity effects of rimonabant, a CB1 antagonist, are mediated through the central or the peripheral CB1 receptors. To address this question, we generated transgenic mice with central nervous system (CNS)-specific knockdown (KD) of CB1, by expressing an artificial microRNA (AMIR) under the control of the neuronal Thy1.2 promoter. In the mutant mice, CB1 expression was reduced in the brain and spinal cord, whereas no change was observed in the superior cervical ganglia (SCG), sympathetic trunk, enteric nervous system, and pancreatic ganglia. In contrast to the neuronal tissues, CB1 was undetectable in the brown adipose tissue (BAT) or the liver. Consistent with the selective loss of central CB1, agonist-induced hypothermia was attenuated in the mutant mice, but the agonist-induced delay of gastrointestinal transit (GIT), a primarily peripheral nervous system-mediated effect, was not. Compared to wild-type (WT) littermates, the mutant mice displayed reduced body weight (BW), adiposity, and feeding efficiency, and when fed a high-fat diet (HFD), showed decreased plasma insulin, leptin, cholesterol, and triglyceride levels, and elevated adiponectin levels. Furthermore, the therapeutic effects of rimonabant on food intake (FI), BW, and serum parameters were markedly reduced and correlated with the degree of CB1 KD. Thus, KD of CB1 in the CNS recapitulates the metabolic phenotype of CB1 knockout (KO) mice and diminishes rimonabant's efficacy, indicating that blockade of central CB1 is required for rimonabant's antiobesity actions.

Induction of fatty acid oxidation resists weight gain, ameliorates hepatic steatosis and reduces cardiometabolic risk factors

OBJECTIVE

Fatty acid oxidation has been implicated in amelioration of obesity by burning off excessive accumulated lipid. BPR697, a peripheral cannabinoid receptor 1 (CB1) antagonist, elevated fat oxidation without added energy expenditure. Its impact on food intake, body weight changes and metabolic alterations were examined in rats fed standard chow and in diet-induced obesity (DIO) mice.

MATERIALS AND METHODS

CB1 agonist-induced hypothermia and analgesia responses were measured to examine the brain activity of BPR697. The acute effects of BPR697 on food intake, body weight change and post-absorptive metabolic profiles were investigated in rats. Energy utilization with BPR697 was examined by indirect calorimetry. Chronic treatment of DIO mice was used to evaluate the long-term effects of BPR697.

RESULTS

Distribution of BPR697 was significantly biased in favor of the periphery instead of the brain, as shown by its low brain/plasma concentration ratio and confirmed by the negative response of BPR697 in CB1 agonist-induced hypothermia and analgesia. When administered to rats at 20 mg kg(-1), BPR697 showed a unique spectrum of effects with significant weight loss without altered food intake. Furthermore, BPR697 increased serum levels of free fatty acids and ketone bodies and reduced hepatic lipid accumulation with preservation of liver glycogen in postprandial rats. Indirect calorimetric profiling of BPR697 revealed a similar trend, shifting whole-body energy catabolism toward fat oxidation, but without elevated energy expenditure. In DIO mice with chronic treatment, animals treated with BPR697 at 20 mg kg(-1) resisted weight gain and showed a reduction of high-fat-induced cardiometabolic abnormalities such as hyperglycemia, abdominal fat and liver steatosis.

CONCLUSION

The induction of fatty acid oxidation without concomitant elevation of energy expenditure by the peripheral CB1 antagonist BPR697 is sufficient to cause substantial weight loss in chow-fed rats. In the presence of high-dietary fat intake, BPR697 resists weight gain and alleviates obesity-related cardiometabolic risk factors.

Peripheral effects of the endocannabinoid system in energy homeostasis: adipose tissue, liver and skeletal muscle

The endocannabinoid system (ECS) is composed of lipid signalling ligands, their G-protein coupled receptors and the enzymes involved in ligand generation and metabolism. Increasingly, the ECS is emerging as a critical agent of energy metabolism regulation through its ability to modulate caloric intake centrally as well as nutrient transport, cellular metabolism and energy storage peripherally. Visceral obesity has been associated with an upregulation of ECS activity in several systems and inhibition of the ECS, either pharmacologically or genetically, results in decreased energy intake and increased metabolic output. This review aims to summarize the recent advances that have been made regarding our understanding of the role the ECS plays in crucial peripheral systems pertaining to energy homeostasis: adipose tissues, the liver and skeletal muscle.

Energy balance regulation by endocannabinoids at central and peripheral levels

Dysregulation of the endocannabinoid system (ECS) is a universal and, perhaps, causative feature of obesity. Central nervous system (CNS) circuits that regulate food intake were initially believed to be the targets for dysregulation. However, it is increasingly evident that endocannabinoids affect food intake, energy expenditure and substrate metabolism by acting on peripheral sites. Cannabinoid type 1 receptor (CB1r) antagonists can effectively treat obesity and associated metabolic alterations but, unfortunately, cause and exacerbate mood disorders. Drugs restricted to act on peripheral CB1rs might be safer and more effective, retaining the anti-obesity effects but lacking the adverse neurodepressive reactions. This review summarizes the emerging roles of the ECS in energy balance and discusses future pharmacological approaches for developing peripherally restricted CB1r antagonists.

The case for peripheral CB₁ receptor blockade in the treatment of visceral obesity and its cardiometabolic complications

In this review, we consider the role of endocannabinoids and cannabinoid-1 (CB(1)) cannabinoid receptors in metabolic regulation and as mediators of the thrifty phenotype that underlies the metabolic syndrome. We survey the actions of endocannabinoids on food intake and body weight, as well as on the metabolic complications of visceral obesity, including fatty liver, insulin resistance and dyslipidemias. Special emphasis is placed on weighing the relative importance of CB(1) receptors located in peripheral tissues versus the central nervous system in mediating the metabolic effects of endocannabinoids. Finally, we review recent observations that indicate that peripherally restricted CB(1) receptor antagonists retain efficacy in reducing weight and improving metabolic abnormalities in mouse models of obesity without causing behavioural effects predictive of neuropsychiatric side effects in humans.

The highs and lows of cannabinoid receptor expression in disease: mechanisms and their therapeutic implications

Alterations in the endogenous cannabinoid system have been described in almost every category of disease. These changes can alternatively be protective or maladaptive, such as producing antinociception in neuropathic pain or fibrogenesis in liver disease, making the system an attractive therapeutic target. However, the challenge remains to selectively target the site of disease while sparing other areas, particularly mood and cognitive centers of the brain. Identifying regional changes in cannabinoid receptor-1 and -2 (CB(1)R and CB(2)R) expression is particularly important when considering endocannabinoid system-based therapies, because regional increases in cannabinoid receptor expression have been shown to increase potency and efficacy of exogenous agonists at sites of disease. Although there have been extensive descriptive studies of cannabinoid receptor expression changes in disease, the underlying mechanisms are only just beginning to unfold. Understanding these mechanisms is important and potentially relevant to therapeutics. In diseases for which cannabinoid receptors are protective, knowledge of the mechanisms of receptor up-regulation could be used to design therapies to regionally increase receptor expression and thus increase efficacy of an agonist. Alternatively, inhibition of harmful cannabinoid up-regulation could be an attractive alternative to global antagonism of the system. Here we review current findings on the mechanisms of cannabinoid receptor regulation in disease and discuss their therapeutic implications.

Central endocannabinoid signaling regulates hepatic glucose production and systemic lipolysis

OBJECTIVE

The endocannabinoid (EC) system has been implicated as an important regulator of energy homeostasis. In obesity and type 2 diabetes, EC tone is elevated in peripheral tissues including liver, muscle, fat, and also centrally, particularly in the hypothalamus. Cannabinoid receptor type 1 (CB₁) blockade with the centrally and peripherally acting rimonabant induces weight loss and improves glucose homeostasis while also causing psychiatric adverse effects. The relative contributions of peripheral versus central EC signaling on glucose homeostasis remain to be elucidated. The aim of this study was to test whether the central EC system regulates systemic glucose fluxes.

RESEARCH DESIGN AND METHODS

We determined glucose and lipid fluxes in male Sprague-Dawley rats during intracerebroventricular infusions of either WIN55,212-2 (WIN) or arachidonoyl-2'-chloroethylamide (ACEA) while controlling circulating insulin and glucose levels through hyperinsulinemic, euglycemic clamp studies. Conversely, we fed rats a high-fat diet for 3 days and then blocked central EC signaling with an intracerebroventricular infusion of rimonabant while assessing glucose fluxes during a clamp.

RESULTS

Central CB₁ activation is sufficient to impair glucose homeostasis. Either WIN or ACEA infusions acutely impaired insulin action in both liver and adipose tissue. Conversely, in a model of overfeeding-induced insulin resistance, CB₁ antagonism restored hepatic insulin sensitivity.

CONCLUSIONS

Thus central EC tone plays an important role in regulating hepatic and adipose tissue insulin action. These results indicate that peripherally restricted CB₁ antagonists, which may lack psychiatric side effects, are also likely to be less effective than brain-permeable CB₁ antagonists in ameliorating insulin resistance.

Endocannabinoids in liver disease

Endocannabinoids are lipid mediators of the same cannabinoid (CB) receptors that mediate the effects of marijuana. The endocannabinoid system (ECS) consists of CB receptors, endocannabinoids, and the enzymes involved in their biosynthesis and degradation, and it is present in both brain and peripheral tissues, including the liver. The hepatic ECS is activated in various liver diseases and contributes to the underlying pathologies. In patients with cirrhosis of various etiologies, the activation of vascular and cardiac CB(1) receptors by macrophage-derived and platelet-derived endocannabinoids contributes to the vasodilated state and cardiomyopathy, which can be reversed by CB(1) blockade. In mouse models of liver fibrosis, the activation of CB(1) receptors on hepatic stellate cells is fibrogenic, and CB(1) blockade slows the progression of fibrosis. Fatty liver induced by a high-fat diet or chronic alcohol feeding depends on the activation of peripheral receptors, including hepatic CB(1) receptors, which also contribute to insulin resistance and dyslipidemias. Although the documented therapeutic potential of CB(1) blockade is limited by neuropsychiatric side effects, these may be mitigated by using novel, peripherally restricted CB(1) antagonists.

Perspectives of CB1 Antagonist in Treatment of Obesity: Experience of RIO-Asia

Rimonabant, a selective cannabinoid-1 (CB1) receptor antagonist, has been shown to reduce weight and enhance improvements in cardiometabolic risk parameters in Western populations. This study assessed these effects of rimonabant in Asian population. A total of 643 patients (BMI 25 kg/m(2) or greater without diabetes) from China, Republic of Korea, and Taiwan were prescribed a hypocaloric diet (600 kcal/day deficit) and randomized to rimonabant 20 mg (n = 318) or placebo (n = 325) for 9months. The primary efficacy variable was weight change from baseline after 9 months of treatment. Results showed that rimonabant group lost more weight than placebo, (LSM ± SEM of -4.7 ± 0.3 kg vs. -1.7 ± 0.3 kg, P < .0001). The 5% and 10% responders were 2 or 3 folds more in the rimonabant group (53.0% vs. 20.0% and 21.5% vs. 5.7%, resp.) (P < .0001). Rimonabant also significantly increased HDL-cholesterol, decreased triglycerides and waist circumference,by 7.1%, 10.6%, and 2.8 cm, respectively (P < .0001). This study confirmed the comparable efficacy and safety profile of rimonabant in Asian population to Caucasians. Owing to the recent suspension of all the CB1 antagonists off the pharmaceutical market for weight reduction in Europe and USA, a perspective in drug discovery for intervening peripheral CB1 receptor in the management of obesity is discussed.

Peripheral CB1 cannabinoid receptor blockade improves cardiometabolic risk in mouse models of obesity

Obesity and its metabolic consequences are a major public health concern worldwide. Obesity is associated with overactivity of the endocannabinoid system, which is involved in the regulation of appetite, lipogenesis, and insulin resistance. Cannabinoid-1 receptor (CB1R) antagonists reduce body weight and improve cardiometabolic abnormalities in experimental and human obesity, but their therapeutic potential is limited by neuropsychiatric side effects. Here we have demonstrated that a CB1R neutral antagonist largely restricted to the periphery does not affect behavioral responses mediated by CB1R in the brains of mice with genetic or diet-induced obesity, but it does cause weight-independent improvements in glucose homeostasis, fatty liver, and plasma lipid profile. These effects were due to blockade of CB1R in peripheral tissues, including the liver, as verified through the use of CB1R-deficient mice with or without transgenic expression of CB1R in the liver. These results suggest that targeting peripheral CB1R has therapeutic potential for alleviating cardiometabolic risk in obese patients.

A selective cannabinoid-1 receptor antagonist, PF-95453, reduces body weight and body fat to a greater extent than pair-fed controls in obese monkeys

Cannabinoid-1 (CB(1)) receptor antagonists exhibit pharmacological properties favorable to treatment of obesity, caused by both centrally mediated effects on appetite and peripherally mediated effects on energy metabolism. However, the relative contribution of these effects to the weight loss produced by CB(1) receptor antagonists remains unclear. Here, we compare food intake-related and independent effects of the CB(1)-selective antagonist 1-(7-(2-chlorophenyl)-8-(4-chlorophenyl)-2-methylpyrazolo[1,5-a][1,3,5]triazin-4-yl)-3-(methylamino) azetidine-3-carboxamide (PF-95453) in obese cynomolgus monkeys. Monkeys were divided into three study groups (n = 10 each) and treated once daily for 8 weeks with either vehicle or PF-95453 as follows: 1, fed ad libitum and dosed orally with vehicle; 2, fed ad libitum and dosed orally with PF-95453 (0.5 mg/kg weeks 1-3, 1.0 mg/kg weeks 4-8); and 3, fed an amount equal to the amount consumed by the drug-treated group and dosed orally with vehicle (pair-fed). PF-95453 treatment significantly reduced food consumption by 23%, body weight by 10%, body fat by 39%, and leptin by 34% while increasing adiponectin by 78% relative to vehicle-treated controls. Pair-fed animals did not exhibit reductions in body weight or leptin but did show significantly reduced body fat (11%) and increased adiponectin (15%) relative to vehicle-treated controls but markedly less than after PF-95453 treatment. Indeed, significant differences were noted between the drug-treated and pair-fed groups with respect to body weight reduction, body fat reduction, increased adiponectin, and leptin reduction. Similar to humans, monkeys treated with the CB(1) receptor antagonist exhibited decreased body weight and body fat, a substantial portion of which seemed to be independent of the effects on food intake.

Alterations in the hippocampal endocannabinoid system in diet-induced obese mice

The endocannabinoid (eCB) system plays central roles in the regulation of food intake and energy expenditure. Its alteration in activity contributes to the development and maintenance of obesity. Stimulation of the cannabinoid receptor type 1 (CB(1) receptor) increases feeding, enhances reward aspects of eating, and promotes lipogenesis, whereas its blockade decreases appetite, sustains weight loss, increases insulin sensitivity, and alleviates dysregulation of lipid metabolism. The hypothesis has been put forward that the eCB system is overactive in obesity. Hippocampal circuits are not directly involved in the neuronal control of food intake and appetite, but they play important roles in hedonic aspects of eating. We investigated the possibility whether or not diet-induced obesity (DIO) alters the functioning of the hippocampal eCB system. We found that levels of the two eCBs, 2-arachidonoyl glycerol (2-AG) and anandamide, were increased in the hippocampus from DIO mice, with a concomitant increase of the 2-AG synthesizing enzyme diacylglycerol lipase-alpha and increased CB(1) receptor immunoreactivity in CA1 and CA3 regions, whereas CB(1) receptor agonist-induced [(35)S]GTPgammaS binding was unchanged. eCB-mediated synaptic plasticity was changed in the CA1 region, as depolarization-induced suppression of inhibition and long-term depression of inhibitory synapses were enhanced. Functionality of CB(1) receptors in GABAergic neurons was furthermore revealed, as mice specifically lacking CB(1) receptors on this neuronal population were partly resistant to DIO. Our results show that DIO-induced changes in the eCB system affect not only tissues directly involved in the metabolic regulation but also brain regions mediating hedonic aspects of eating and influencing cognitive processes.