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 (CB 1 R) 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 CB 1 R neutral antagonist largely restricted to the periphery does not affect behavioral responses mediated by CB 1 R 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 CB 1 R in peripheral tissues, including the liver, as verified through the use of CB 1 R-deficient mice with or without transgenic expression of CB 1 R in the liver. These results suggest that targeting peripheral CB 1 R has therapeutic potential for alleviating cardiometabolic risk in obese patients. IntroductionEndocannabinoids are endogenous lipid mediators that interact with the same G protein-coupled receptors - CB 1 R and CB 2 R - that recognize plant-derived cannabinoids, and they regulate a broad range of physiological functions. CB 1 Rs are expressed at very high levels in the brain but are also present at much lower yet functionally relevant concentrations in various peripheral tissues, whereas the expression of CB 2 Rs is largely limited to cells of the immune and hematopoietic systems. Activation of CB 1 R results in increased appetite, insulin resistance, and increased hepatic lipogenesis, which suggests the involvement of the endocannabinoid/ CB 1 R system in obesity and its metabolic consequences (1). Indeed, obesity and its metabolic complications are characterized by an overactive endocannabinoid system (2-5), and chronic treatment with CB 1 R antagonists leads to weight loss and improved cardiometabolic risk profile in obese rodents (6, 7) and humans (8-11). However, concern over neuropsychiatric side effects, including anxiety, depression, and suicidal ideation (12), prevented approval of the first-in-class CB 1 R antagonist rimonabant in the United States and led to its withdrawal from the European market as well as the withdrawal of related compounds from preclinical development (13). Although the exact role of the endocannabinoid system in the control of mood and anxiety-like behaviors is not clear, CB 1 R in the prefrontal cortex, amygdala, and the mesolimbic dopaminergic reward pathway have been linked to the control of these behaviors (14). On the other hand, CB 1 Rs are also present in peripheral tissues including the liver (15-17), skeletal muscle (18,19), endocrine

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 (CB 1 R) 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 CB 1 R neutral antagonist largely restricted to the periphery does not affect behavioral responses mediated by CB 1 R 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 CB 1 R in peripheral tissues, including the liver, as verified through the use of CB 1 R-deficient mice with or without transgenic expression of CB 1 R in the liver. These results suggest that targeting peripheral CB 1 R has therapeutic potential for alleviating cardiometabolic risk in obese patients. IntroductionEndocannabinoids are endogenous lipid mediators that interact with the same G protein-coupled receptors - CB 1 R and CB 2 R - that recognize plant-derived cannabinoids, and they regulate a broad range of physiological functions. CB 1 Rs are expressed at very high levels in the brain but are also present at much lower yet functionally relevant concentrations in various peripheral tissues, whereas the expression of CB 2 Rs is largely limited to cells of the immune and hematopoietic systems. Activation of CB 1 R results in increased appetite, insulin resistance, and increased hepatic lipogenesis, which suggests the involvement of the endocannabinoid/ CB 1 R system in obesity and its metabolic consequences (1). Indeed, obesity and its metabolic complications are characterized by an overactive endocannabinoid system (2-5), and chronic treatment with CB 1 R antagonists leads to weight loss and improved cardiometabolic risk profile in obese rodents (6, 7) and humans (8-11). However, concern over neuropsychiatric side effects, including anxiety, depression, and suicidal ideation (12), prevented approval of the first-in-class CB 1 R antagonist rimonabant in the United States and led to its withdrawal from the European market as well as the withdrawal of related compounds from preclinical development (13). Although the exact role of the endocannabinoid system in the control of mood and anxiety-like behaviors is not clear, CB 1 R in the prefrontal cortex, amygdala, and the mesolimbic dopaminergic reward pathway have been linked to the control of these behaviors (14). On the other hand, CB 1 Rs are also present in peripheral tissues including the liver (15-17), skeletal muscle (18,19), endocrine

Drugs that interfere with cannabinoid CB1 transmission suppress various food-motivated behaviors, and it has been suggested that such drugs could be useful as appetite suppressants. Biochemical studies indicate that most of these drugs assessed thus far have been CB1 inverse agonists, and although they have been shown to suppress food intake, they also appear to induce nausea and malaise. The present studies were undertaken to characterize the behavioral effects of AM4113, which is a CB1 neutral antagonist, and to examine whether this drug can reduce food-reinforced behaviors and feeding on diets with varying macronutrient compositions. Biochemical data demonstrated that AM4113 binds to CB1 receptors, but does not show inverse agonist properties (ie no effects on cyclic-AMP production). In tests of spontaneous locomotion and analgesia, AM4113 reversed the effects of the CB1 agonist AM411. AM4113 suppressed food-reinforced operant responding with rats responding on fixed ratio (FR) 1 and 5 schedules of reinforcement in a dosedependent manner, and also suppressed feeding on high-fat, high-carbohydrate, and lab chow diets. However, in the same dose range that suppressed feeding, AM4113 did not induce conditioned gaping, which is a sign of nausea and food-related malaise in rats. These results suggest that AM4113 may decrease appetite by blocking endogenous cannabinoid tone, and that this drug may be less associated with nausea than CB1 inverse agonists. Neuropsychopharmacology (2008) 33, 946-955;

Mesolimbic dopamine (DA) is involved in behavioral activation and effort-related processes. Rats with impaired DA transmission reallocate their instrumental behavior away from food-reinforced tasks with high response requirements, and instead select less effortful food-seeking behaviors. In the present study, the effects of several drug treatments were assessed using a progressive ratio (PROG)/chow feeding concurrent choice task. With this task, rats can lever press on a PROG schedule reinforced by a preferred high-carbohydrate food pellet, or alternatively approach and consume the less-preferred but concurrently available laboratory chow. Rats pass through each ratio level 15 times, after which the ratio requirement is incremented by one additional response. The DA D2 antagonist haloperidol (0.025–0.1 mg/kg) reduced number of lever presses and highest ratio achieved but did not reduce chow intake. In contrast, the adenosine A2A antagonist MSX-3 increased lever presses and highest ratio achieved, but decreased chow consumption. The cannabinoid CB1 inverse agonist and putative appetite suppressant AM251 decreased lever presses, highest ratio achieved, and chow intake; this effect was similar to that produced by pre-feeding. Furthermore, DA-related signal transduction activity (pDARPP-32(Thr34) expression) was greater in nucleus accumbens core of high responders (rats with high lever pressing output) compared to low responders. Thus, the effects of DA antagonism differed greatly from those produced by pre-feeding or reduced CB1 transmission, and it appears unlikely that haloperidol reduces PROG responding because of a general reduction in primary food motivation or the unconditioned reinforcing properties of food. Furthermore, accumbens core signal transduction activity is related to individual differences in work output.

BACKGROUND AND PURPOSE Cannabinoid CB1 receptor antagonists reduce food intake and body weight, but clinical use in humans is limited by effects on the CNS. We have evaluated a novel cannabinoid antagonist (AM6545) designed to have limited CNS penetration, to see if it would inhibit food intake in rodents, without aversive effects. EXPERIMENTAL APPROACH Cannabinoid receptor binding studies, cAMP assays, brain penetration studies and gastrointestinal motility studies were carried out to assess the activity profile of AM6545. The potential for AM6545 to induce malaise in rats and the actions of AM6545 on food intake and body weight were also investigated. KEY RESULTS AM6545 binds to CB1 receptors with a Ki of 1.7 nM and CB2 receptors with a Ki of 523 nM. AM6545 is a neutral antagonist, having no effect on cAMP levels in transfected cells and was less centrally penetrant than AM4113, a comparable CB1 receptor antagonist. AM6545 reversed the effects of WIN55212‐2 in an assay of colonic motility. In contrast to AM251, AM6545 did not produce conditioned gaping or conditioned taste avoidance in rats. In rats and mice, AM6545 dose‐dependently reduced food intake and induced a sustained reduction in body weight. The effect on food intake was maintained in rats with a complete subdiaphragmatic vagotomy. AM6545 inhibited food intake in CB1 receptor gene‐deficient mice, but not in CB1/CB2 receptor double knockout mice. CONCLUSIONS AND IMPLICATIONS Peripherally active, cannabinoid receptor antagonists with limited brain penetration may be useful agents for the treatment of obesity and its complications.

CB receptor coupling to opposing G proteins is determined by both receptor and G protein expression levels, which underpins a mechanism for non-canonical signalling in a fashion consistent with Gα signalling. CB receptors mediate opposite consequences in endpoints such as tumour viability depending on expression levels; our results may help to explain such effects at the level of G protein coupling.

Behavioral activation is a fundamental feature of motivation, and organisms frequently make effort-related decisions based upon evaluations of reinforcement value and response costs. Furthermore, people with major depression and other disorders often show anergia, psychomotor retardation, fatigue, and alterations in effort-related decision making. Tasks measuring effort-based decision making can be used as animal models of the motivational symptoms of depression, and the present studies characterized the effort-related effects of the vesicular monoamine transport (VMAT-2) inhibitor tetrabenazine. Tetrabenazine induces depressive symptoms in humans, and also preferentially depletes dopamine (DA). Rats were assessed using a concurrent progressive ratio (PROG)/chow feeding task, in which they can either lever press on a PROG schedule for preferred high-carbohydrate food, or approach and consume a less-preferred lab chow that is freely available in the chamber. Previous work has shown that the DA antagonist haloperidol reduced PROG work output on this task, but did not reduce chow intake, effects that differed substantially from those of reinforcer devaluation or appetite suppressant drugs. The present work demonstrated that tetrabenazine produced an effort-related shift in responding on the PROG/chow procedure, reducing lever presses, highest ratio achieved and time spent responding, but not reducing chow intake. Similar effects were produced by administration of the subtype selective DA antagonists ecopipam (D1) and eticlopride (D2), but not by the cannabinoid CB1 receptor neutral antagonist and putative appetite suppressant AM 4413, which suppressed both lever pressing and chow intake. The adenosine A2A antagonist MSX-3, the antidepressant and catecholamine uptake inhibitor bupropion, and the MAO-B inhibitor deprenyl, all reversed the impairments induced by tetrabenazine. This work demonstrates the potential utility of the PROG/chow procedure as a rodent model of the effort-related deficits observed in depressed patients.

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