Treat Endocrinol 2004; 3 (6): 345-360 1175-6349/04/0006-0345/$31.00/0
LEADING ARTICLE
2004 Adis Data Information BV. All rights reserved.
Endocannabinoid Receptor Antagonists Potential for Obesity Treatment Tim C. Kirkham1 and Claire M. Williams2 1 2
School of Psychology, The University of Liverpool, Liverpool, UK School of Psychology, University of Reading, Reading, UK
Abstract
Obesity has been described as a global epidemic. Its increasing prevalence is matched by growing costs, not only to the health of the individual, but also to the medical services required to treat a range of obesity-related diseases. In most instances, obesity is a product of progressively less energetic lifestyles and the over-consumption of readily available, palatable, and highly caloric foods. Past decades have seen massive investment in the search for effective anti-obesity therapies, so far with limited success. An important part of the process of developing new pharmacologic treatments for obesity lies in improving our understanding of the psychologic and physiologic processes that govern appetite and bodyweight regulation. Recent discoveries concerning the endogenous cannabinoids are beginning to give greater insight into these processes. Current research indicates that endocannabinoids may be key to the appetitive and consummatory aspects of eating motivation, possibly mediating the craving for and enjoyment of the most desired, most fattening foods. Additionally, endocannabinoids appear to modulate central and peripheral processes associated with fat and glucose metabolism. Selective cannabinoid receptor antagonists have been shown to suppress the motivation to eat, and preferentially reduce the consumption of palatable, energy-dense foods. Additionally, these agents act to reduce adiposity through metabolic mechanisms that are independent of changes in food intake. Given the current state of evidence, we conclude that the endocannabinoids represent an exciting target for new anti-obesity therapies.
There is a growing prevalence of obesity in Western society, and increasingly also in those regions of the world with developing industrial economies.[1,2] Obesity is associated with a wide spectrum of diseases, such as type 2 diabetes mellitus, cardiovascular morbidity, and cancer, which together constitute a significant and growing burden on our health resources.[3-5] For the most part, the underlying causes of this global rise in bodyweight and its associated problems lie in our evolutionary history and cultural development; there are only very rare occurrences of genetic anomalies with phenotypes that include overeating or significant accretion of adipose stores.[6-9] Evolution and the pre-agricultural struggle for survival programmed our species to maximize all opportunities to eat, to be susceptible to food variety, to derive especial pleasure from high-energy foods, and to be particularly efficient at storing excess calories as fat in preparation for times of famine. Modern cultures are exemplified by progressively more sedentary lifestyles and, while daily caloric intake may not always differ dramatically from that of 30 or 40 years ago, the balance between energy intake and expenditure has certainly shifted substantially
toward a level of daily consumption that may far exceed our bodies’ caloric requirements.[6,7] In addition, as levels of physical exercise have fallen, changing eating styles have accommodated the over-consumption of a wide variety of manufactured foods that appeal to our pre-programmed susceptibility to temptation; meals are all too regularly supplemented by snacks that in themselves would provide sufficient calories for our daily needs. Increasingly across the world, and most notably in the US and the UK, chronic over-consumption of readily available, calorically dense, and highly palatable foods has already resulted in a significant proportion of the population becoming overweight or obese. While logic tells us that the prevalent imbalance between energy expenditure and consumption could be best attacked through encouraging more exercise, the natural motivation to eat and the pleasure derived from eating is rather more compelling than the satisfaction to be derived from greater physical activity. Indeed, for many people, the lifetime accumulation of adipose stores itself represents a significant obstacle to weight reduction through exercise, even when combined with restricted consump-
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tion. Consequently, there have been considerable efforts to develop alternative methods of reducing the weight of the morbidly obese, largely through pharmacologic treatments aimed at reducing the individual’s motivation to eat. Such anti-obesity treatments represent a potential multi-billion dollar market for the pharmaceutical industry. However, despite extensive research programs dedicated to this field over the past few decades, it is apparent that few effective treatments have resulted.[10] In this article, we review recent developments in the field of cannabinoid-appetite research, and consider the potential of cannabinoid receptor antagonists in the treatment of obesity. 1. Pharmacologic Approaches to the Treatment of Obesity Initially, appetite-suppressant treatments consisted essentially of stimulants such as the amphetamines. However, the abuse potential of such drugs naturally became an obstacle to their prescription.[10,11] Amongst the more successful compounds was an amphetamine derivative, dexfenfluramine, which was found to suppress appetite while being largely devoid of the abuse potential of the stimulants.[12] Research from the 1970s onwards indicated that dexfenfluramine exerted its psychologic actions by facilitating brain serotonergic activity associated with physiologic processes mediating the satiation, or termination, of eating.[13,14] Dexfenfluramine (and the neurochemically less selective d,l-fenfluramine) had a short commercial life due to occasionally fatal adverse effects.[15] Nevertheless, the effectiveness of dexfenfluramine spawned research into other serotonergic compounds that can suppress eating by promoting feelings of fullness, thereby limiting meal size. Recently, emphasis has shifted to selective serotonin reuptake inhibitors.[10] For example, fluoxetine can suppress appetite and promote weight loss in obese individuals.[16] However, these beneficial effects appear to be transient, with longterm treatments failing to sustain weight loss or even inducing weight gain.[17] Augmenting noradrenergic function has also been linked to weight loss, and a popular anorectic strategy was the combination of fenfluramine with the noradrenergic reuptake inhibitor phentermine.[10] Latterly, this approach has been exploited with the availability of combined serotonin-norepinephrine reuptake inhibitors.[18] A recently approved anti-obesity drug in this class is sibutramine, which combines appetite suppression with beneficial thermogenic effects; but health concerns may also restrict its use.[19] More generally, research has primarily targeted an increasing list of putative hormonal and neurochemical signals believed to promote the satiation of eating and feelings of satiety.[20] The variety of these candidate satiety signaling targets, many of them peptides, is too large for extensive discussion in this 2004 Adis Data Information BV. All rights reserved.
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article, but so far none has led to effective clinical applications.[10,21,22] 2. Targeting Appetitive Processes In the light of the biologic imperatives to eat already discussed (and one’s own, everyday experience), the prevailing notion that our bodies and brains employ multiple, redundant, physiologic satiation signals to limit our intake may seem misconceived. If such factors were effective in restraining eating, it is unlikely that obesity would now constitute the problem that it does. At best, one might argue that if the multiplicity of putative satiety signals do have a true physiologic role in appetite control, then they may be necessary to confine the fundamental and prepotent drive to ensure nutritional integrity. Similarly, orthodox homeostatic conceptualizations of energy balance, whereby food intake is regulated by systems that monitor consumption and the accretion of fat stores, may not adequately model actual human feeding behavior and weight change. As opportunistic, omnivorous feeders, humans continuously demonstrate their ability to over-consume – even to the point of discomfort – in the face of a tantalizing array of palatable foods. Animal experiments also provide many instances of the failure of satiation models. For example, preventing the entry of food into the stomach (using esophageal fistulae), or the passage of food from the stomach into the intestinal tract (by pyloric occlusion or gastric fistulae), does not necessarily alter the normal patterning or structure of meals.[23-25] Sham feeding studies (in which food is recovered directly after ingestion, before it can reach receptors in the gut that might be linked to the generation of peripheral satiety signals) indicate the power and primacy over intake of psychologic factors such as conditioning and the immediate, pleasurable orosensory properties of food.[23,24,26,27] For example, in normal laboratory rats, simple manipulations that enhance food palatability (such as the addition of water to a powdered laboratory diet to make a paste) can provoke daily consumption levels that far exceed those normally observed with the routine, dry, relatively bland diet, even though the nutritional complement of the two diets are identical.[28] In this instance, in spite of the considerable over-consumption of the more palatable food, the mechanisms proposed to monitor the passage and absorption of nutrients along the gastrointestinal tract fail to impose any compensatory control over eating. Availability of a desirable food is thus sufficient to generate counter-regulatory over-consumption. The simple deduction from such observations is that the pleasure derived from the taste, texture, flavour, and even the temperature of food (orosensory reward in jargon terms) is by far the most influential determinant of intake. Importantly, what is often deTreat Endocrinol 2004; 3 (6)
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scribed as a simple process of satiation – from the initial impulse to eat, to the point of no longer being willing to eat a given food or meal – does not give rise to an incontrovertible state of satiety.[29,30] If we add variety, more food will be eaten than if only a single food item is provided. Even after a large meal, further provision of a particularly palatable food will engender yet more eating (the ‘dessert effect’). Additionally, we are all familiar with the ability of the sight or smell of a favorite food to engender a strong desire to eat even though eating may previously have been far from our thoughts.[31] These factors render simple satiety signal models somewhat less than adequate to account for human feeding behavior and our tendency to over-consume. In view of these considerations we can see the limitations of drug treatments designed specifically to facilitate satiation processes. Unless accompanied by strict dietary control or behavior modification, satiety-enhancing drugs may not prevent the easy over-consumption of calories from a few mouthfuls of forbidden foods. Moreover, if temptation cannot be resisted, the initial positive orosensory experience of a palatable food will accentuate our appetite and promote further consumption. Sadly, we need eat only a few hundred calories more than we expend each day for significant long-term bodyweight accumulation to occur, or to negate the generally modest weight loss produced by recently prescribed appetite suppressants.[10] Thus, we might seek a more effective focus for weight-reducing pharmacotherapy: our innate responsiveness to the incentive and sensory properties of food may well be the key. It is in this context that the latest developments in cannabinoid pharmacology are of particular interest, specifically the availability of the cannabinoid receptor antagonist rimonabant, which appears to possess a novel profile of potentially therapeutic properties.[32] In the following sections we will address the evidence relating endogenous cannabinoid systems to the physiologic control of appetite, and discuss the potential of cannabinoid interventions to assist in weight-loss programs. 3. Cannabis and Endogenous Cannabinoids Humankind has been familiar with the psychotropic and medicinal actions of Cannabis sativa (marijuana) for thousands of years, but it is only in the past decade that real progress has been made in understanding the cellular mechanisms underlying those actions.[33] The psychoactive compounds contained in cannabis were first characterized in 1964, when Gaoni and Mechoulam[34] isolated ∆9-tetrahydrocannabinol (THC) and a group of related ‘cannabinoid’ molecules.[35] Subsequently, it was demonstrated that these cannabinoids exert their effects via specific binding sites within the CNS and peripheral tissues. We now know that there is 2004 Adis Data Information BV. All rights reserved.
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a family of G-protein-linked, cell-surface cannabinoid receptors.[36-38] Two main cannabinoid receptor subtypes have been identified.[39] These are classified as a ‘central-type’ CB1 receptor, widely distributed within the CNS and many peripheral tissues, and a ‘peripheral-type’ CB2 receptor, which is not significantly expressed in the CNS.[40] It is generally agreed that the behavioral effects of cannabinoids are mediated by brain (CB1) cannabinoid receptors and, despite their wide distribution, the regional localization of receptors corresponds closely with their behavioral effects.[41,42] The existence of specific receptor sites that can mediate the effects of plant-derived exogenous cannabinoids indicated the existence of chemicals produced within mammalian tissues, for which the cannabinoid receptors are the target. That is, endogenous ligands that compounds such as THC can mimic in order to induce their various effects. The 1990s saw the isolation of the first ‘endocannabinoid’.[43] This compound, arachidonylethanolamide, which is synthesized within brain tissue and binds with high affinity to CB1 receptors, was named anandamide – from ‘ananda’, a Sanskrit word meaning inner bliss. Subsequently, the search for additional endogenous ligands selective for the CB2 cannabinoid receptor led to the identification of 2-arachidonoyl glycerol (2-AG).[44,45] Although it exhibits a lower affinity for CB1 receptors than anandamide, evidence suggests that 2-AG is present in the brain at higher levels than anandamide and is a full agonist at CB1 receptors.[45] Both of these substances fulfill the necessary criteria for classification as neuromodulators: they are synthesized from arachidonic acid through distinct biosynthetic routes; are released from neurons in response to membrane depolarization and calcium influx; have specific uptake mechanisms; and are hydrolyzed by a selective fatty acid amide hydrolase.[46-48] Cannabinoids are also closely related to the arachidonic acid-derived eicosanoids, and may have overlapping physiologic functions.[49,50] Other candidate endocannabinoids have since been characterized, including noladin ether and virodhamine,[51-53] but anandamide and 2-AG are considered to be the primary ligands at CB1 and CB2 receptors, with both substances capable of exerting THC-like effects in animal behavioral models.[43,54] Importantly, amphibian, rodent, and human CB1 receptors show a high degree of homology. Together with the occurrence of the endocannabinoids in a number of phylogenetically diverse species, a high degree of evolutionary conservation of cannabinoid signaling systems indicates that they should play an important physiologic role in vertebrate brain function.[55,56] Treat Endocrinol 2004; 3 (6)
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4. Cannabinoids and the Stimulation of Appetite In searching for the physiologic roles of the endogenous cannabinoids, there are many clues from the long-documented accounts of the effects of marijuana. Amongst the wide spectrum of marijuana’s psychologic effects is the tendency of the drug to stimulate eating and create what can be described as a voracious appetite, known colloquially by cannabis users as ‘the munchies’.[57] Following the characterization of the exogenous, plant-derived cannabinoids, it was soon demonstrated that THC was a particularly potent ‘orexigenic’ agent. This capacity to stimulate eating ultimately led to the drug’s clinical application in the treatment of appetite loss and wasting in patients with cancer and AIDS.[58-60] We are still awaiting well controlled, systematic analyses of the psychologic and physiologic correlates of the hyperphagic action of THC in humans, not least because of the continuing political and legal impediments that discourage research into cannabis. However, there is now sufficient evidence to indicate that cannabinoid-induced eating reflects the involvement of endocannabinoids in the normal regulation of appetite and feeding behavior. In addressing the empirical evidence from human and animal studies, we will initially focus on data that specifically relate to the motivational factors described in section 2 (food incentive and orosensory reward) and that represent rational targets for new antiobesity treatments. Consistent themes that emerge in the marijuana-feeding literature are those of enhanced desire to eat, increased sensitivity to the sensory properties of foods, and apparently preferential effects on preferred, highly palatable foods (although as described below, it is now clear that food does not have to be very palatable for cannabinoid hyperphagia to be induced).[57] In 1970, Tart[61] described marijuana users’ subjective accounts, noting such descriptions as: “taste sensations take on new qualities”; “I enjoy eating very much and eating a lot”; “if I try to imagine what something tastes like, I can do so very vividly”; “I crave sweet things to eat, like chocolate, more than other foods”. In one of the first controlled studies with THC, Hollister[62] examined its acute effects on the consumption of chocolate milk shakes. The drug significantly increased intake, consistently elevated hunger ratings, and enhanced food appreciation. Similarly, albeit in a less well controlled experiment, Abel[63] observed that inhalation of cannabis cigarettes led individuals to eat as many as 50 marshmallows, compared with only four by control individuals. A series of more systematic studies into marijuana-induced eating have been conducted by Foltin et al.,[64,65] using a relatively naturalistic, residential laboratory in which volunteers were housed for periods of up to 25 days.[66] In addition to consistently 2004 Adis Data Information BV. All rights reserved.
observing substantial increases in daily caloric intake after cannabis smoke inhalation, these workers reported that this excess was primarily obtained by an increase in the frequency and consumption of snack foods, rather than of the set meals provided each day. Specifically, with ad libitum food access, marijuana increased total food intake by doubling the number of snacks, particularly of sweet, solid items such as candy bars, cookies, and cakes. The intake of sweet drinks (e.g. cola, fruit juice) or savoury solid items (e.g. potato chips) was less affected. Similar effects of THC on food selection have been reported by Mattes et al.,[67] with increases in energy intake derived principally from increased snack consumption rather than self-selected meals. Greater light has been thrown on the motivational specificity of cannabinoids through the use of animal models. The most commonly used laboratory species, the rat, is a useful model as it has similar opportunistic, omnivorous tendencies and responsivity to tasty foods as do humans. In one of the first studies to demonstrate cannabinoid hyperphagia in rats, Brown et al.[68] found that low, orally administered doses of THC increased the intake of food and particularly a palatable sucrose solution. Significantly, AndersonBaker et al.[69] reported that hyperphagia could be induced by direct injection of THC into the lateral or ventromedial hypothalamic nuclei of freely feeding rats, brain areas that are heavily implicated in the physiologic regulation of appetite.[70] More recently, we demonstrated that oral THC was able to induce particularly potent effects in animals that were thoroughly satiated by voluntary consumption of large amounts of palatable food prior to drug administration. Thus, the most effective dose of THC (1 mg/kg) produced a >4-fold increase in consumption over a 1-hour test.[28] This effect was far greater than any previously reported with the drug, and comparable to the effects of the most potent orexigenic compounds previously reported. The ‘pre-satiation’ procedure was intended to ensure low baseline intake levels and so maximize our ability to detect drug-induced eating, but the potency of THC in this model has particular significance. Firstly, it reinforces our earlier remarks about the capacity of humans (or rats) to eat, even in the face of repletion and positive energy balance. Well satiated, THC-treated animals not only over-consumed compared with control animals, but their intake levels were actually comparable to those typically seen in hungry, fooddeprived rats. Secondly, as the test food in these experiments was an ordinary, bland maintenance diet, our effects demonstrate the compelling nature of cannabinoid-induced eating, and indicate the probable importance of endocannabinoid systems in the normal instigation of appetite and eating motivation, irrespective of the food type. Supporting that role, our subsequent experiments have confirmed that THC hyperphagia is mediated by central CB1 cannabinoid receptors, as it is attenuated by the selective CB1 Treat Endocrinol 2004; 3 (6)
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receptor antagonist rimonabant (SR141716), but not by SR144258, a selective antagonist of the peripheral CB2 receptor.[71] The notion of endocannabinoid involvement in the normal, physiologic regulation of appetite was further strengthened by studies demonstrating that acute or chronic CB1-receptor blockade can reliably suppress food intake in laboratory animals (see below), suggesting that tonic endocannabinoid activity may be a key component in the neurochemical regulation of appetite.[72-74] This possibility was crucially supported by our demonstration that peripheral administration of the endogenous cannabinoid anandamide also stimulated feeding, albeit less potently than THC.[75] Moreover, anandamide hyperphagia was prevented by rimonabant, while the CB2 antagonist SR144258 was without effect, showing that the overeating was specifically mediated by central-type CB1 receptors. Subsequently, anandamide-induced eating in rodent models has been confirmed by other groups, with very low peripheral doses[76,77] and after direct administration into the ventromedial nucleus of the hypothalamus.[78] More recently, we demonstrated that another endocannabinoid, 2-AG, will also induce eating in free-feeding rats, after either peripheral injection or bilateral infusion into the shell region of the nucleus accumbens (AcbSh).[79] Additionally, both anandamide and 2-AG will promote feeding when administered into the lateral hypothalamus.[57,80] Importantly, both the hypothalamus and the AcbSh are brain regions that are firmly associated with eating motivation,[70,81] and their sensitivity to the hyperphagic actions of anandamide and 2-AG provides further confirmation of a key role for endocannabinoids in the control of eating. 5. Anorectic Actions of Cannabinoid Receptor Antagonists Complementing the actions of cannabinoid receptor agonists are data showing that cannabinoid-receptor blockade can suppress feeding. The ability of a CB1 receptor antagonist to alter feeding behavior by blocking the actions of endogenous agonists clearly implicates endocannabinoid-mediated events in the normal expression of feeding behavior. Soon after the isolation of CB1 cannabinoid receptors several selective antagonists were developed. One of the most influential of these has been the CB1 receptor antagonist rimonabant, originally synthesized by RinaldiCarmona and colleagues.[32] As already described, rimonabant will attenuate the hyperphagic actions of exogenous and endogenous cannabinoids.[75,78,79] However, even before endocannabinoid-induced feeding had been demonstrated, acute peripheral administration of the drug was shown to reduce food intake in laboratory animals.[72-74] More recently, reliable anorectic actions of rimonabant, or its analog AM281, have been reported following intra 2004 Adis Data Information BV. All rights reserved.
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cerebroventricular administration in satiated or food-deprived rats.[82] An important aspect of early studies of rimonabant was an apparent action of the drug to preferentially attenuate the intake of preferred, palatable foods – an effect observed in both rats and marmosets.[72,73] It should be noted that, although palatable foods are generally more susceptible to the anorectic actions of rimonabant and its analogs, CB1 receptor antagonists are also effective in reducing the intake of less appetizing foods, such as the nutritionally balanced but bland diets fed to laboratory animals (commonly referred to as ‘lab chow’).[74,82,83] This fact supports the wider generality of endocannabinoid involvement in feeding regulation but, as we shall see in section 7, the greater susceptibility of palatable foods to CB1-receptor blockade has very specific implications for understanding the role of endocannabinoids in motivational processes. In addition to acute treatments, several chronic studies have been reported in which systemic antagonist treatments have induced reliable intake suppression. In most instances, tolerance to the drugs’ anorectic actions appears to develop after several days. However, these treatments do produce significant, and persistent, reductions in bodyweight that are at least partly a consequence of the initial reduction in food intake.[74,83-85] These studies and their implications for anti-obesity applications are discussed in more depth in sections 10 and 11. Overall, the consequences of CB1-receptor blockade on feeding behavior in these experiments suggest that tonic release of endocannabinoids is influential in controlling eating in adult animals, but there are also indications that cannabinoid processes may be crucial to feeding in the earliest developmental stages. Thus, Fride and colleagues[86-88] have provided evidence that the initiation of suckling in neonates is cannabinoid dependent. Following the discovery that anandamide and 2-AG are present in animal and human milk,[89] these workers investigated the influence of endocannabinoids on suckling behavior. They demonstrated that CB1receptor knockout mice fail to suckle during the first day after birth, and that rimonabant administration to healthy mouse pups prevents suckling during the same critical 24-hour period, an effect with potentially fatal consequences that can be reversed by administration of cannabinoid receptor agonists.[86-88] A recent study by Matias et al.[90] has further demonstrated that maternal nutritional status during gestation and lactation can significantly affect neonatal levels of brain endocannabinoids. Thus, restriction of essential polyunsaturated fatty acids in the maternal diet, and specifically of the endocannabinoid precursor arachidonic acid, was shown to result in reduced hypothalamic anandamide levels in the brains of weaning rat pups. Given the dependence of developing animals on polyunsaturated fatty acids from maternal blood Treat Endocrinol 2004; 3 (6)
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during gestation, and milk in neonatal stages, weaning rats in this study were likely to suffer from low availability of anandamide precursors. An associated reduction in the bodyweights of these animals might therefore imply deficits in feeding, and is again suggestive of an important role for endocannabinoids in the regulation of ingestive behavior.
6. Motivational Targets of Cannabinoids: Interactions with Hunger and Food Palatability As we have already emphasized, the subjective effects of cannabis or THC in humans, and some aspects of the animal data, have led to the speculation that endocannabinoids are implicated in the hedonic evaluation of ingesta. Put simply, it has been argued that the pleasure we obtain from food is mediated to some significant extent by the activity of brain endocannabinoids.[73] The reader is cautioned that this hypothesis was initially proposed on the basis of less than extensive animal experimentation, and was largely justified by essentially anecdotal accounts of the effects of cannabis in humans. Just as we have criticized the prevailing notions of satiety mechanisms and energy homeostasis, we urge caution in extrapolating too far from the changes to food intake, often measured over arbitrary intervals, in animal models after CB1 receptor agonist or antagonist administration. There is only so much that can be inferred from the measurement of changes in the weight of food remaining in rat cages after 1 hour or 24 hours; and, at best, inferences made from animal experiments about the motivational specificity of cannabinoid actions must be constrained by the limited experimental contexts. Since we are still awaiting detailed analyses of the psychologic correlates of cannabinoid interactions with appetite, food palatability, and the psychophysical aspects of taste in people, current hypotheses are necessarily tentative. Nevertheless, there are strong indications from the recent literature on animal experiments to suggest very specific involvement of endocannabinoids in the motivational processes that direct and energize eating behavior. Current conceptions of eating motivation consider there to be two principal and separable components that govern food seeking and actual consumption: ‘wanting’ and ‘liking’.[91] These components respectively relate to the anticipation of, or craving for, food and its potentially rewarding properties, or the actual emotional experience, or pleasure, arising from its ingestion. The hypotheses about cannabinoids and feeding discussed in section 4 generally emphasize the liking aspect of eating, arguing for enhanced palatability of food after cannabinoid administration. However, there is growing evidence for a key role of endocannabinoids in the anticipatory, wanting aspect of eating motivation. 2004 Adis Data Information BV. All rights reserved.
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Early evidence that cannabinoid interventions modify appetitive-incentive processes was provided by Gallate and McGregor,[92] who found that rats will work harder to obtain palatable ingesta after administration of CB1 receptor agonists.[93,94] Conversely, antagonist treatments attenuate responding for food or palatable solutions. Using more detailed behavioral analysis, we observed the open-field behavior of satiated rats following administration of THC and anandamide.[95] Under control conditions rats generally displayed little motivation to eat, with eating occurring only after many minutes engaged in exploratory behaviors. By contrast, both exogenous and endogenous cannabinoid treatments stimulated feeding almost as soon as food became available. Crucially, once initiated, the subsequent pattern of feeding behavior displayed by cannabinoid-treated rats in the open field is identical to that of untreated rats feeding freely in their home cages.[57,95] Similarly, examination of the meal patterns of freely feeding rats after peripheral or central treatment with anandamide or 2-AG reveals that meal onset is consistently advanced.[57,95,96] Such findings imply that stimulation of CB1 receptors enhances the salience or incentive value of food, and hence increases the motivation to approach food and begin eating. We thus begin to see the development of a model that links endocannabinoids directly to the processes that lead to the generation of appetite and the initiation of feeding. To some extent, the effect of cannabinoid receptor agonists on eating latency – the apparently greater urgency to eat after cannabinoid administration – are similar to those that might be seen in food-deprived animals. In line with this proposition, CB1-receptor knockout mice are distinguished from their wild-type littermates by a reduced hyperphagic response to fasting.[97] Further support for the notion comes from our examination of interactions between food deprivation and the anorectic potency of rimonabant on lab chow intake. The antagonist was administered to rats that had been food deprived for 18 hours or maintained on a schedule of restricted access to food. We found that, whereas ad libitum fed rats were only weakly affected by low doses of the drug, restricting food availability resulted in significant intake suppression by rimonabant.[57] As rimonabant acts as a competitive antagonist at CB1 receptors, the behavioral effects of CB1-receptor blockade are likely to become apparent only if there is endogenous cannabinoid release and receptor stimulation. Thus, the greater the level of cannabinoid activity, the greater may be the behavioral consequences of rimonabant treatment. Werner and Koch[82] have confirmed the susceptibility of ordinary lab chow intake to CB1-receptor blockade in food-deprived rats. In fact, they found that intracerebroventricular injection of the CB1 antagonist AM281 could produce almost complete suppression of eating. Treat Endocrinol 2004; 3 (6)
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These data thus suggest that deprivation induces, or enhances, endocannabinoid activity. We therefore hypothesized that endogenous cannabinoid activity may normally increase during the intervals between meals (and particularly after deprivation), to reach some critical level at which point motivation to eat is triggered. Accordingly, the longer the time that has elapsed since the last meal, the greater will be the activity in relevant endocannabinoid circuits, and the higher the motivation to eat. This proposition received support from subsequent experiments involving direct measures of brain endocannabinoid levels. We examined regional brain anandamide and 2-AG levels in animals after food deprivation, while consuming a palatable food, or after satiating on that food.[79] Endocannabinoid levels in these animals were compared against those from nondeprived rats, at a point during their daily feeding cycle when motivation to eat was minimal and feeding absent. We reasoned that if endocannabinoid activity is key to the appetitive phase of eating motivation then food deprivation, with its obvious ability to provoke hunger, would maximize our ability to detect relevant changes in anandamide and 2-AG. Alternatively, if endocannabinoid activity were to contribute more directly to orosensory reward during ingestion, allowing animals to avidly consume a highly palatable food should maximize relevant changes. Finally, examining the brains of animals that had over-consumed a palatable food to the point of satiation provided an additional control to indicate the specific involvement of endocannabinoids during all stages of appetite and eating. We found that acute food deprivation provoked significant increases in brain anandamide and 2-AG levels. These effects were most marked in the limbic forebrain, with substantial increases in the levels of both endocannabinoids. Within the hypothalamus, only 2-AG was reliably increased. That the two cannabinoids were differentially regulated probably reflects the high degree of anatomical complexity in the distribution of appetiterelated, cannabinoid-sensitive neurons. In particular, the hypothalamus contains a number of zones (e.g. lateral hypothalamic, paraventricular, ventromedial, and arcuate nuclei) that are separately implicated in the stimulation or inhibition of feeding, as well as the regulation of energy metabolism.[70,98] Support for these findings comes from a recent study by Hanus et al.[99] who observed elevated 2-AG levels in whole mouse brains after acute food deprivation for 24 hours. However, that group also found reduced hippocampal and hypothalamic 2-AG levels in mice that were chronically food restricted (to 40–60% of their ad libitum intake over 12 days). These latter changes were interpreted as an adaptation to semi-starvation, rather than to an acute state of hunger as imposed in our experiments. 2004 Adis Data Information BV. All rights reserved.
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The deprivation-induced changes in endocannabinoid levels that we observed are likely to reflect the specific activation of neural systems involved in the appetitive components of eating motivation, as opposed to those responsible for the active maintenance of eating once food becomes available. The other significant change in hypothalamic 2-AG that we detected may also support this proposition. Specifically, we found that in nondeprived animals given access to palatable food and sacrificed while still highly motivated to continue eating, hypothalamic 2-AG levels were actually significantly reduced. By contrast, in animals that ate the same food to satiety, hypothalamic 2-AG levels were restored to levels similar to those in our control animals, measured at a time when they were replete and their endogenous feeding rhythms naturally suppressed eating.[79] The respective elevation of hypothalamic 2-AG levels with deprivation, and the decline evident during feeding suggest that, once initiated, eating is no longer dependent on endocannabinoid activity for its maintenance. Indeed, there may be specific mechanisms to suppress 2-AG activity during re-feeding to facilitate satiation. Certainly, our observation of reduced hypothalamic 2-AG levels in animals eating a palatable diet indicates that endocannabinoids are not crucial to food palatability – the orosensory reward that guides the rate, duration, and size of meals. A substantial increase in endocannabinoid levels in response to caloric deficit was also apparent within the limbic forebrain, in this case, for both 2-AG and anandamide. The magnitude of these deprivation-induced increases (and an absence of changes with any other manipulation) suggests activation of circuitry with specific involvement in the generation of appetite and food-seeking behavior. Notably, this region contains the AcbSh, an important component of brain reward pathways.[81] 7. Endocannabinoids and Incentive-Reward Processes Central to these reward pathways are the mesolimbic dopaminergic neurons, arising in the ventral tegmental area and projecting to the nucleus accumbens.[100] Natural rewards, including food, together with many drugs of abuse, have been found to stimulate dopamine release from terminals in the nucleus accumbens. Researchers now emphasize a specific role for these pathways in incentive motivation, i.e. the generation of emotional arousal and behavioral activation in response to stimuli, which predict reward.[91,100,101] Ingestion of food causes dopamine release in the nucleus accumbens, especially after deprivation, or if the food is novel or palatable. In addition, food restriction is known to enhance the rewarding properties of food and of drugs of abuse.[102,103] It is perhaps not coincidental, then, that doses of Treat Endocrinol 2004; 3 (6)
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THC that we have found to produce hyperphagia have also been found to stimulate dopamine release in the nucleus accumbens.[28,104] The AcbSh, which contains a relatively high density of CB1 receptors,[41,105] has particularly strong associations with appetitive processes. Neural activity in the shell is believed to signal incentive salience, and to facilitate the generation of motor patterns orienting an animal toward potentially rewarding stimuli, such as food in a hungry animal.[81,106] Given this association, it is notable that we have found that the AcbSh is a particularly sensitive site for the induction of cannabinoid-induced feeding. Specifically, we have shown that feeding can be induced by direct administration of 2-AG into this region of the rat brain, with substantial short-term increases in food intake being observed.[79] As with anandamide and THC, 2-AG significantly advances the onset of feeding. Moreover, the rapidity of onset and magnitude of 2-AG hyperphagia when injected into the AcbSh far outweigh the relatively weak effects of anandamide so far seen after peripheral or central administration.[75-78] Although the data described above indicate a primary role for endocannabinoids in appetitive, incentive, or ‘wanting’ aspects of eating motivation, we will now address the possibility of cannabinoid influences on the consummatory, ‘liking’ component of feeding. Recently, we examined this issue directly by examining the effects of CB1 receptor ligands on the microstructure of sucrose drinking, a technique that permits the direct comparison of drug effects on ingestive responses with those produced simply by altering the palatability of sucrose (by varying its concentration).[107] Crucially, we found that both exogenous and endogenous cannabinoids altered drinking in a way that mimics the effect of increasing sucrose palatability. Conversely, the actions of rimonabant on licking microstructure replicated the effects of sucrose dilution, indicating that the drug effectively reduced the reward value of the sucrose solution. We have argued that the apparent dual actions of cannabinoids on appetitive and consummatory components of ingestive behavior are not incompatible. Certainly, motivation systems that are designed to increase the salience of food stimuli, encourage food seeking, and initiate ingestion would be less than effective if they were not paired with activation of those systems responsible for maintaining intake once food becomes available. The activation of systems subserving wanting, together with the priming of liking motivation components makes sense in terms of the biologic imperatives discussed in the introduction. Thus, the behavioral actions of cannabinoids may be indicative of a general activation of incentive-reward circuitry that both instigates feeding and enables a greater appreciation of food once eating commences. The precise nature of cannabinoid mechanisms in the intra-meal he 2004 Adis Data Information BV. All rights reserved.
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donic evaluation of food remains to be determined, but we believe that they may involve alterations to the activity of endogenous opioid systems in the brain. In addition to dopamine, the endogenous opioid peptides are also linked to central reward processes. For example, in the accumbens, dopamine neurons synapse with enkephalinergic neurons that are critical to the processing of reward.[104] Evidence has accumulated to support overlapping endogenous opioid and endocannabinoid mechanisms in relation to a wide range of physiologic processes, including reward and appetite.[108] For example, CB1-receptor knockout mice are not only unresponsive to cannabinoids, but display a reduced sensitivity to the rewarding properties of opiate drugs.[109] Importantly, Gallate and McGregor[92] found that the facilitatory effects of a CB1 receptor agonist on responding for palatable solutions were reversed by both a CB1 receptor antagonist and the general opioid receptor antagonist, naloxone. In addition, THC-induced Fos immunoreactivity in appetite-related brain regions can be modified by naloxone.[110] Such findings again imply that cannabinoids may modulate the motivation to ingest via actions on both endocannabinoid and opioid systems. Such a link may be anticipated since opioids are firmly implicated in the mediation of food reward. Opioid receptor agonists and antagonists respectively increase or reduce food intake by altering the hedonic evaluation of foods during ingestion. For example, opioid antagonists are reported by humans to reduce the perceived palatability of normally preferred foods and fluids.[111,112] We have obtained convincing evidence for interactions between cannabinoids and endogenous opioids in relation to feeding. For example, we found that even low, subanorectic doses of naloxone effectively blocked cannabinoid-induced over-consumption.[71] We also examined whether combined administration of the CB1 receptor antagonist rimonabant with naloxone could provide further evidence of co-operative interactions between cannabinoid and opioid systems.[113] We chose a range of doses of each antagonist, which alone is capable of reversing the actions of selective agonists at their respective binding sites but exert no significant effect on lab chow intake. Thus, neither naloxone alone, nor rimonabant alone, produced any reliable effects on food intake. However, when given in combination, every dose of rimonabant potentiated the effects of all doses of naloxone. Significant intake suppression occurred with every combination of the two drugs, relative to controls (vehicle only, vehicle/rimonabant, and vehicle/ naloxone).[113] Similarly, supra-additive effects of rimonabant and naloxone have also been demonstrated by Rowland and colleagues.[114] Those data seem to indicate a synergistic interaction between the effects of opioid and cannabinoid receptor antagonists, and go Treat Endocrinol 2004; 3 (6)
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a long way toward supporting an important functional relationship between cannabinoid and opioid systems in the normal regulation of appetite. Apparent effects of cannabinoid receptor ligands on palatability responses are therefore likely to involve some direct influence on the opioid systems that contribute to the instantaneous hedonic evaluation of ingesta. Whatever the actual mechanisms, these findings do strengthen the role of endocannabinoids in the processes that underlie our sensitivity to, and over-consumption of palatable foods. A noteworthy corollary to these data comes from recent findings by Harrold et al.[115] that also link the consumption of palatable food to endocannabinoid activity. They compared regional CB1-receptor density in the brains of rats maintained on lab chow with those of animals given a palatable food supplement for 10 weeks. Over-consumption and accelerated weight gain in the palatable food group resulted in the downregulation of CB1 receptors, with receptor density reduced by as much as 50% in some areas, including within the nucleus accumbens and other areas associated with the hedonic aspects of eating. Moreover, the extent of downregulation was correlated with the extent of over-consumption of the palatable diet. These authors concluded that this effect was consistent with increased activation of these receptors by endocannabinoids, and that anandamide and 2-AG may drive the appetite for palatable foods. In the light of those findings, it is interesting to note the finding by Poncelet et al.[116] that, in comparison to normal animals, CB1-receptor –/– mice show a reduced hyperdipsic response to a palatable, sucrose solution. 8. Endocannabinoids and Hypothalamic Integration Surprisingly, the study by Harrold et al.[115] failed to detect any change in CB1-receptor density in the hypothalamus, which is classically associated with food intake and bodyweight regulation. This result would suggest that hypothalamic cannabinoids are not involved in the processes controlling palatable food intake. Possibly, this failure may be related to the fact that hypothalamic CB1receptor levels are relatively low compared with other regions.[105] However, within the hypothalamus, higher CB1-receptor expression is evident within the lateral hypothalamic nuclei that are linked to mechanisms that initiate feeding, and that are also anatomically and functionally associated with the nucleus accumbens.[106,117] Moreover, it has been found that CB1-receptor coupling to G-proteins is more efficient within the hypothalamus than in brain regions with higher receptor densities.[42] Despite these complexities, the hypothalamus has been the focus of several investigations which have examined potential relationships between cannabinoids and a range of hypothalamic neuropeptides that have intake-stimulating (‘orexigenic’) or in 2004 Adis Data Information BV. All rights reserved.
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take-suppressing (‘anorexigenic’) actions. Orexigenic peptides include neuropeptide Y, orexins, melanin-concentrating hormone, and galanin. Anorexigenic neuropeptides include α-melanocytestimulating hormone, corticotropin-releasing hormone (CRH), and cocaine and amphetamine-related transcript.[20,70] Current homeostatic models propose that hypothalamic orexigenic and anorexigenic neuropeptides are regulated by peripheral signals related to energy status, such as the peptide hormone leptin that is secreted by adipocytes in proportion to the level of stored fat.[118,119] It is therefore of interest that there is evidence that hypothalamic anandamide and 2-AG biosynthesis may be subject to negative regulation by leptin. Thus, leptin administration suppresses hypothalamic levels of endocannabinoids in normal rats, while in genetically obese, chronically hyperphagic rats and mice with deficient leptin signaling, the elevated hypothalamic expression of anandamide or 2-AG is reversible in response to leptin.[97] Of the several hypothalamic nuclei-assigned roles in the regulation of energy balance, the lateral hypothalamus is of particular interest. This region has reciprocal neural links with the nucleus accumbens, and may be critical to the translation of the motivation to eat into actual feeding behavior, and as a component of the brain’s incentive-reward systems, to the hedonic evaluation of food.[81,106] Chemical or electrical stimulation of the lateral hypothalamus induces intense feeding activity even in satiated animals, apparently by raising the incentive value of food stimuli through the activation of reward circuitry.[106,120] Importantly, appetiteinducing manipulations within the nucleus accumbens (which is a sensitive site for 2-AG-induced eating) may induce the coordinated activation of hypothalamic neurons that express orexigenic peptides, and the suppression of those neurons expressing anorexigenic peptides.[121] Lateral hypothalamic feeding circuits are also sensitive to cannabinoid modulation. For example, THC facilitates feeding induced by electrical stimulation of this region, reducing the threshold level of stimulation required to induce the behavior.[122] Electrical stimulation of the brain also mimics the reward associated with eating, and rats will work to obtain such stimulation alone. Interestingly, the threshold level at which rats will respond for lateral hypothalamic stimulation is reduced by food restriction.[123] Thus, in line with our earlier arguments about cannabinoid actions on feeding resembling those of food deprivation, THC and fasting exert similar effects. Conversely, leptin, which will naturally have higher circulating levels in replete animals,[124] has been found to elevate stimulation thresholds.[125] Importantly, rimonabant, which reduces the motivation to eat, has been shown to have a similar effect to leptin.[126] Such data suggest that leptin may act, through the downregulation of endocannabinoids, to reduce the general incentive value of food and so restrict feeding. Food deprivation and the concomitant Treat Endocrinol 2004; 3 (6)
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suppression of circulating leptin levels would therefore be expected to release endocannabinoids from this control, with increased hypothalamic levels (as in our study[79]) and a consequent increase in the attractiveness of food. Moreover, in spontaneously feeding human volunteers, circulating leptin levels have been shown to increase during inter-meal intervals and decline before the onset of a meal.[124] It is therefore possible to predict a reciprocal relationship between leptin and hypothalamic endocannabinoids in the regulation of meal patterns. It should be noted, however, that some workers have questioned whether leptin plays a role in controlling cannabinoid-mediated intake of palatable food. For example, there appears to be no correlation between plasma leptin levels and brain CB1-receptor expression in normal animals that become obese and hyperleptinemic through the overconsumption of a palatable diet.[115] More research is obviously required to resolve these contradictions. Recently, Hilairet and coworkers[127] obtained evidence of functional interactions between endocannabinoids and orexin A, an orexigenic peptide that is selectively expressed in the lateral hypothalamus.[128,129] More specifically, evidence was obtained for cross-talk between CB1 receptors and the orexin receptor OX1R, such that activation of CB1 receptors can enhance orexin signaling.[127] Together with this group’s finding that rimonabant can block orexin A-induced feeding, these data indicate that there may be a range of important functional associations between endocannabinoids and other putative central appetite control systems. Indeed, other workers have detected potentially significant relationships between cannabinoids and appetite-related hypothalamic peptides. Thus, Cota et al.[130] found that CB1 receptors are coexpressed with CRH, amphetamine-related transcript, preproorexin, and melanin-concentrating hormone in mouse hypothalamic neurons. Additionally, CB1-receptor knockout mice (which are characterized by hypophagia, reduced bodyweight, and reduced fat mass compared with their wild-type littermates) show higher levels of mRNA for the anorexigen CRH. These findings may indicate a possible oppositional relationship between hypothalamic cannabinoids and CRH in the normal regulation of appetite.[130] Recalling our discussion of functional relationships between opioids and cannabinoids, there is also evidence for an important relationship between these neuromodulators within the hypothalamus, and particularly the paraventricular nucleus (PVN). The PVN is the focus of converging orexigenic and anorexigenic neuropeptide pathways, and is believed to be crucial to the hypothalamic integration of metabolic, hormonal, and neural factors regulating energy homeostasis.[70,131] Importantly, the PVN is a sensitive site for the hyperphagic actions of cannabinoid receptor agonists. Moreover, in addition to CB1 receptors, opioid receptors are also 2004 Adis Data Information BV. All rights reserved.
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expressed within the PVN.[105,132] It is significant, therefore, that Verty et al.[133] have recently reported that feeding induced by direct injection of morphine into this site can be reversed by rimonabant. The full implications of these data require further investigation, but they again reinforce the intimate relationship between endocannabinoids and intake regulatory mechanisms at many levels within the brain. 9. A Role for Peripheral Cannabinoid Systems? The preceding discussion seems to provide strong evidence that the alterations to feeding induced by CB1 receptor ligands are mediated through actions on central processes. Clearly, the psychologic representation and behavioral expression of altered eating motivation require processing within the brain, particularly in situations where the incentive or reward value of food is evaluated. Nevertheless, it has been argued that the feeding actions of systemically administered CB1 receptor ligands predominantly involve peripheral sites of action. Thus, G´omez et al.[77] compared the hyperphagic and anorectic potencies of anandamide and rimonabant when administered intraperitoneally or intracerebroventricularly. Contrary to the positive findings of several other groups noted above, G´omez et al. observed drug effects only after peripheral but not central injection. Moreover, the actions of systemic anandamide and rimonabant were abolished by capsaicin-induced deafferentiation of peripheral sensory nerves, suggesting a possible mechanism whereby stimulation/blockade of peripheral CB1 receptors may influence central motivation processes. These workers also demonstrated that anandamide is synthesized within intestinal tissues, with increased levels after 24 hours of fasting.[77] These results were interpreted as indicating a possible role for peripheral anandamide as a hunger signal. However, in contrast to reports by other groups showing elevated brain endocannabinoid levels in starved animals,[79,99] these workers detected no change in brain anandamide levels. While the findings by G´omez et al. do support potentially important contributions of peri-pheral cannabinoid systems to feeding regulation, the inconsistencies relating to the reported effectiveness of centrally administered CB1 ligands and brain anandamide levels need to be resolved by further investigation. 10. The Potential of Cannabinoid Antagonists as Anti-Obesity Treatments The preceding sections have highlighted the accumulating evidence for a role of endocannabinoids in the complex mechanisms that regulate food intake. Combining our comments on the principal factors that contribute to obesity and the apparent involvement of endocannabinoids in incentive and reward processes, it is not Treat Endocrinol 2004; 3 (6)
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unreasonable to consider that the anorectic actions of CB1 receptor antagonists have potential in the treatment of obesity. Several pharmaceutical companies are actively pursuing this possibility. Indeed, phase III clinical trials with rimonabant are already being conducted in patients with obesity,[134] with preliminary reports suggesting significant benefits in terms of food intake and weight reduction. In line with the animal data and our hypotheses relating to appetitive processes, the drug is reported to reduce subjective ratings of hunger.[135] In developing cannabinoid antagonist treatments for obesity, the results of chronic effects of rimonabant are obviously of importance. In an early study, Colombo and colleagues[74] demonstrated that daily administration of the antagonist suppressed appetite for lab chow and induced persistent weight loss in rats. Although tolerance to the drug’s effect on appetite was apparent after 5 days, suppression of bodyweight gain was evident across the full course of a 14-day experiment. More recently, Vickers et al.[83] demonstrated that sub-chronic, oral treatment with the antagonist dose-dependently decreased food (lab chow) intake and bodyweight gain in both lean and genetically obese Zucker (fa/fa) rats. Once again, tolerance to the intake-reducing effects was evident after 4 days in lean animals, but the reduction of bodyweight gain was maintained over 28 days. The inhibition of food intake and bodyweight gain was greater in obese Zucker rats than in lean control rats, with daily food intake initially reduced by as much as 40% and persistent weight loss evident over the first 2 weeks of treatment. Moreover, the development of tolerance to the drug’s anorectic action was considerably delayed (until day 13) in obese animals, although their intake subsequently remained lower than in vehicle-treated control animals. Even after tolerance developed, the rate of weight gain remained significantly suppressed in antagonist-treated rats. The withdrawal of rimonabant on day 28 resulted in rebound hyperphagia and significant weight gain. As mentioned earlier, obese Zucker rats have elevated hypothalamic levels of 2-AG.[97] It is therefore possible that differences in endocannabinoid tone within the hypothalamus could account for the increased effectiveness of rimonabant in these animals. Both of these studies indicate that rimonabant can chronically attenuate the intake of relatively bland laboratory diets. This effect is contrary to some of the data discussed in section 5 that pointed to selective effects of the antagonist on palatable food intake. Nevertheless, the chronic effectiveness of rimonabant on relatively bland food intake points to the possibility that the drug would be useful in reinforcing the restraint of obese patients maintained on relatively unpalatable, calorically restricted diets. However, a better test of the intake- and weight-attenuating potential of the drug is whether it can be effective in suppressing the intake of palatable 2004 Adis Data Information BV. All rights reserved.
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foods. Administration of appetite suppressants in obese individuals would normally be paired with dietary restriction of some kind. However, there is always the potential for individuals to succumb to the attractions of favorite foods, and maintaining a strict dietary regimen, even with the aid of drugs, is a substantial obstacle to the maintenance of weight loss. This issue has been more directly addressed by recent studies of the effects of CB1 receptor antagonists in mice made obese through the provision of a high-fat diet. This diet-induced obesity is a good model for the most common form of human obesity and its consequences, including visceral obesity and type 2 diabetes. Ravinet-Trillou et al.[84] examined the chronic actions of rimonabant in dietary obese mice. Once again, daily administration of the drug induced a substantial early reduction of energy intake, with food intake reduced by almost 50% during the first week of a 5-week study. This initial suppressive effect gradually waned, but intake remained suppressed compared with vehicletreated control mice throughout the whole test period. Overall, bodyweights were reduced substantially after 1 week and stabilized at that lower level until the end of the experiment. Posttreatment carcass analysis showed that rimonabant significantly reduced adipose stores, halving the proportion of body fat seen in control mice fed the same high-fat diet, while preserving lean mass. A follow-up study confirmed these effects and demonstrated that the drug-induced changes in food intake and body composition were dose dependent. Additionally, elevated plasma levels of insulin and leptin, and insulin resistance that accompanied the development of obesity (and which are also features of human obesity[136]) were substantially reduced by antagonist treatment. Hildebrandt and colleagues[85] confirmed the general effectiveness of chronic CB1-receptor blockade in diet-induced obesity using another CB1 receptor antagonist, AM251 (a close structural analog of rimonabant[137]). In their hands, the drug dosedependently suppressed food intake, initially by as much as 60% below control levels. Significant dose-related weight loss was evident after 3 days of treatment, and was maintained over an initial 2-week period of daily oral administration, with significant intake suppression evident until day 12. Hyperphagia was apparent during a subsequent drug-free inter-treatment phase, but bodyweight gain was negligible until after 4–5 drug-free days. Reliable anorexia and weight loss was reinstated when AM251 treatment was given for a further 2 weeks, again with very marked initial suppression of food intake. In this second-drug stage, tolerance to the intake-suppressing actions of the drug were evident somewhat earlier (day 7), but weight loss was maintained, resulting in marked overall reductions in adipose tissue mass. Plasma leptin and cholesterol levels were also significantly reduced at the highest dose, with an additional tendency for plasma insulin levels to Treat Endocrinol 2004; 3 (6)
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be reduced. The ability of AM251 to attenuate eating and accelerate weight loss with an interrupted administration regimen suggests that tolerance to the effects of CB1 receptor antagonists may not be an obstacle to the long-term application of these drugs in the treatment of obesity. 11. Cannabinoid Receptor Antagonists and Metabolic Factors Each of these chronic studies has noted that weight loss persists after marked anorectic effects of the antagonist have subsided. Although it is possible that cumulative weight loss predominantly reflects the early, marked intake reduction, Ravinet-Trillou et al.[84] provided evidence for an additional action of the antagonists on metabolic processes. For example, pair-feeding tests, in which control animals receive the same amount of food voluntarily consumed by the antagonist-treated animals, showed that weight loss was greater with rimonabant.[84] In other words, although both groups ate the same amount, antagonist-treated animals lost significantly more weight. Additionally, when deprived of food for 24 hours, rimonabant-treated obese mice lost more weight than similarly deprived control mice, and displayed fasting glycemia and insulin sensitivity that were similar to those in lean animals.[84] The combined behavioral and metabolic changes induced by CB1-receptor blockade may reflect alterations to the hypothalamic integrative systems discussed in section 8, to affect not only feeding motivation but also the processes controlling energy homeostasis. For example, the PVN has been ascribed a critical role in the autonomic control of energy balance as part of the hypothalamic-pituitary-adrenal axis, through its sensitivity to the actions of glucocorticoids secreted by the adrenal cortex.[138] Recently Di et al.[139] obtained evidence that the established orexigenic actions of glucocorticoids[140] in the PVN may be mediated by endocannabinoids, possibly through the inhibition of anorexigenic peptides such as CRH. Given that weight loss in animals treated chronically with CB1 receptor antagonists persists after the initial anorectic actions of the drug have dissipated, there may be other important metabolic factors that contribute to their actions on bodyweight. One possibility is that rimonabant may enhance fatty acid oxidation, since it was found to lower plasma-free fatty acid levels in dietary obese mice.[84] Similarly, the reported effects of rimonabant to correct hyperglycemia, reduce plasma insulin levels, and counter insulin resistance suggest that the drug may also improve glucose homeostasis. Additionally, rimonabant may act via hypothalamic mechanisms to increase sympathetic nervous system activity that stimulates lipolysis.[84] These propositions may be extended by the results of recent in vivo studies by Cota et al.,[130] who found that 2004 Adis Data Information BV. All rights reserved.
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not only are CB1 receptors expressed by adipocytes, but also that agonist stimulation of these receptors will dose-dependently stimulate lipogenesis. Interestingly, Bensaid et al.[141] subsequently reported that CB1-receptor expression is upregulated in adipocytes of obese Zucker rats (relative to the levels observed in lean littermates) and during differentiation of cultured mouse adipocytes. The findings by Cota et al. suggest that rimonabant treatment may cause direct interference with cannabinoid-mediated processes that regulate fat deposition in adipose tissues, raising the possibility of important anti-obesity actions of cannabinoid receptor antagonists that are mediated by both central and peripheral mechanisms. Recently, Bensaid and colleagues[141] described a possible route by which the food intake-independent actions of rimonabant on bodyweight could be mediated – involving cannabinoid-sensitive adipose factors. They specifically examined the role of a protein exclusively expressed and secreted by adipose tissue, adiponectin (or adipocyte complement-related protein; Acrp30).[142-145] Plasma levels of the protein and levels of adiponectin mRNA have been shown to vary inversely with adiposity in animals and humans.[146] Moreover, adiponectin has a number of effects that closely match those of rimonabant. Thus, adiponectin has been found to regulate hyperglycemia, hyperinsulinemia, and fatty acid oxidation.[147-149] Importantly, systemic adiponectin administration will also reduce the bodyweight of obese animals through a mechanism that is independent of food intake levels.[150] Replicating the earlier chronic studies,[83,84] Bensaid et al.[141] examined the effects of daily administration of rimonabant in obese Zucker rats. As before, the drug produced an initial, short-lasting anorexia and marked, persistent weight reduction. After 4 days of treatment, adiponectin mRNA expression was significantly increased, with greater effects apparent with longer periods of treatment. A similar action was also observed in lean Zucker rats, but to a lesser extent and with a slower rate of onset. In vitro studies with cultured mouse adipocytes showed that rimonabant stimulation of adiponectin mRNA expression was CB1-receptor mediated, since no effect of the drug was apparent in adipocytes from CB1-receptor knockout mice. Paralleling the rimonabant-induced changes to adiponectin mRNA expression and associated weight loss, rimonabant also reduced the hyperinsulinemia that is characteristic of obesity. It remains to be determined the extent to which these experiments have unmasked important peripheral mechanisms by which endocannabinoids can affect bodyweight regulation. However, the ability of rimonabant to regulate hormones linked to fat and glucose metabolism, promote weight loss, and suppress food intake raises intriguing possibilities for this class of drugs in the treatment of obesity. Treat Endocrinol 2004; 3 (6)
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Despite the emphasis of this review on cannabinoid receptormediated processes in relation to appetite and bodyweight, an interesting corollary to the preceding discussion is found in recent work on a natural analog of anandamide, oleoylethanolamide (OEA), which is synthesized within the gut.[77] Like anandamide, OEA is produced in cells in a stimulus-dependent manner and is rapidly eliminated by enzymatic hydrolysis, suggesting a function in cellular signaling. Although OEA does not activate cannabinoid receptors and its biologic functions are still unknown, Rodriguez de Fonseca and colleagues[77,151] have proposed that intestinal OEA may play a role in the peripheral components of feeding regulation, and particularly on satiation processes. Thus, OEA synthesis in the small intestine is stimulated by feeding and inhibited by food deprivation, and OEA reduces food intake in free-feeding and starved animals, primarily by delaying the onset of meals.[151,152] Additionally, OEA can respectively attenuate the feeding actions of cannabinoids or enhance rimonabant anorexia.[77] Recent research suggests that OEA actions on feeding and bodyweight may be mediated through adiponectin-related mechanisms. Thus, OEA has been found to be an agonist of the peroxisome-proliferator-activated receptor-α (PPARα), a subtype of nuclear hormone receptor.[153] Both OEA and synthetic PPARα agonists reduce food intake and bodyweight in animal models, with comparable alterations to serum lipids.[153] Clearly, there are obvious parallels between these effects and the actions of adiponectin. Indeed, a physiologic link may be evidenced by the fact that stimulation of PPARα has also been shown to alter adiponectin expression in genetically obese db/db mice (although, problematically, both reduced and enhanced adiponectin expression have been observed).[154-156] Interestingly, exogenous cannabinoids, their metabolites, and synthetic analogs have been reported to bind to PPARγ receptors,[157] stimulation of which is known to increase adiponectin expression in obese mice and insulin-resistant obese humans.[158] The full implications of these findings remain to be elucidated, but the possibility of significant interactions between cannabinoid receptor ligands and factors regulating lipid and glucose metabolism suggests potentially fruitful avenues for pharmaceutical treatments of obesity. 12. Conclusion The data presented here summarize the latest developments in the accelerating field of cannabinoid-appetite research. There is now overwhelming evidence that endocannabinoids are important to the regulation of appetite. More specifically, there is strong evidence that these neuromodulators play a role in orienting us toward food stimuli, by enhancing the incentive value or salience 2004 Adis Data Information BV. All rights reserved.
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of foods. Stimulation of central CB1 cannabinoid receptors can provoke feeding even in the face of nutritional repletion. Additionally, there is evidence that activation of endocannabinoid systems affecting appetitive motivation can also produce a positive gain in the activity of other systems that mediate food palatability, such as those utilizing the endogenous opioids. These combined functions suggest a crucial role for endocannabinoids in both stimulating and maintaining appetite, particularly for palatable foods. Furthermore, a growing body of data links endocannabinoids to other components of the complexing range of putative neural and hormonal systems implicated in feeding regulation and energy homeostasis. Importantly, this research is helping to shift the focus of academic and industrial research efforts from ‘satiety signals’ toward the long-neglected, but behaviorally more influential neural underpinnings of hunger, incentive processes, and orosensory reward. As we argued earlier, the modern problem of obesity is a product of our capacity to desire and enjoy foods that possess an intrinsic attractiveness derived from their energy-providing components. Evolutionary pressures, combined with cultural ingenuity in devising appetizing cuisines and modern societal pressures, conspire to provide us with all the gastronomic delights we could desire, but also a surfeit that far exceeds our physical needs. The developments we have described indicate that endocannabinoid systems, by potentially mediating our food cravings and pleasures, are a promising – and probably highly effective – target for pharmacologic interventions designed to control the kinds of eating that are least susceptible to current therapies. Thus, we have seen how cannabinoid antagonists may preferentially suppress the intake of the most palatable, highly caloric foods. Additionally, these drugs seem to have enhanced potency under conditions of hunger induced by food restriction, such as might commonly apply with calorie-controlled diets in obese patients. In effect, cannabinoid receptor antagonists may facilitate weight loss by countering the most potent psychologic factors underlying over-consumption and obesity. The further possibility that drugs such as rimonabant can also have beneficial actions on glucose or fat metabolism provides for even broader, more effective therapeutic possibilities. Of course, there is much that remains to be understood, and many complex issues to be resolved. But we are still in the early stages of what should prove to be an exciting chapter in the psychology and physiology of appetite, and of medicinal approaches to the global epidemic of obesity. Acknowledgments T.C. Kirkham is supported by a grant from the UK BBSRC. The authors have no conflicts of interest that are directly relevant to the content of this review. Treat Endocrinol 2004; 3 (6)
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Correspondence and offprints: Professor Tim C. Kirkham, School of Psychology, The University of Liverpool, Eleanor Rathbone Building, Bedford Street South, Liverpool, L69 7ZA, UK. E-mail:
[email protected] Treat Endocrinol 2004; 3 (6)
