Low dose anandamide affects food intake, cognitive function, neurotransmitter and corticosterone levels in diet-restricted mice

Central dysregulations in the control of energy homeostasis and endocrine alterations in anorexia and bulimia nervosa

In the last decades we have come to understand that the hypothalamus is a key region in controlling energy homeostasis. A number of control models have been proposed to explain the regulation of feeding behavior in physiological and pathological conditions, but all those based on imbalances of single factors fail to explain the disrupted regulation of energy supply in eating disorders such as anorexia nervosa and bulimia nervosa, as well as other psychiatric disorders. A growing amount of evidence demonstrates that many signaling molecules originated within the brain or coming from the adipose tissue or the gastro-enteric tract are involved in the highly complex process controlling food intake and energy expenditure. The recent discovery of leptin, ghrelin, and other factors have made it possible to penetrate in the still undefined pathophysiology of eating disorders with the hope of finding effective treatments for such diseases.

Grooming, scratching and feeding: role of response competition in acute anorectic response to rimonabant in male rats

RATIONALE

Although the CB1 receptor antagonist/inverse agonist rimonabant acutely suppresses food intake in rodents, the behavioural specificity of this effect remains unclear.

OBJECTIVES

To profile the behavioural effects of rimonabant in a free-feeding context.

MATERIALS AND METHODS

Videoanalysis was employed to characterise the effects of acute rimonabant (1.5 and 3.0 mg/kg, IP) on the behaviour of non-deprived male rats exposed to palatable mash. Data were also collected on post-treatment weight gain, and, as prolonged appetite suppression has been found after single dosing with compounds of this series, rats were reassessed (drug-free) for food intake 7 days after initial testing.

RESULTS

Both doses of rimonabant not only decreased mash consumption (44-55%) but also reduced 24-h weight gain. Although videoanalysis confirmed the inhibitory effects of rimonabant on feeding behaviour, it also revealed concurrent reductions in locomotion, rearing and sniffing as well as substantial (up to tenfold) and dose-dependent increases in grooming and scratching. Timecourse analyses further revealed that rimonabant dose-dependently induced frequent episodes of atypical scratching that waned over the test but which were succeeded by prolonged and behaviourally disruptive grooming. Finally, as groups did not differ in mash consumption on retest, any prolonged anorectic effect of acute rimonabant dissipates within 7 days of treatment.

CONCLUSIONS

The anorectic response to rimonabant in male rats would appear to be due largely to response competition. This parsimonious conclusion is supported by the less profound (although still significant) increases in scratching and grooming observed in rats treated with a sub-anorectic dose (0.5 mg/kg) of the compound.

Acute anorectic response to cannabinoid CB1 receptor antagonist/inverse agonist AM 251 in rats: indirect behavioural mediation

Despite a large and consistent literature on the suppressant effects of cannabinoid CB1 receptor antagonists/inverse agonists (e.g. rimonabant, AM 251) on food intake and weight gain in rodents, surprisingly little is known about the behavioural selectivity of such effects. In this study, ethological scoring was used to characterize the acute behavioural effects of the rimonabant analogue AM 251 (1.5 and 3.0 mg/kg, intraperitoneally) in nondeprived male rats during a 1-h test with palatable mash. Data were also collected on daily weight gain and on retest food intake 7 days after dosing. Results showed that the higher dose of AM 251 significantly inhibited mash consumption (32% decrease relative to vehicle control), reduced time spent feeding during the test and suppressed body weight gain over the 48-h period that followed acute dosing. No effects on mash consumption were observed when the animals were retested drug-free 1 week after drug treatment. Detailed video analysis of the test sessions showed that, over the dose range tested, AM 251 did not significantly interfere with the vast majority of noningestive behaviours. Both doses of the compound, however, significantly increased the incidence of and the time spent on scratching, whereas the higher dose additionally increased both the number and duration of grooming episodes. The latter effect in particular disrupted the normal structure of behaviour (behavioural satiety sequence) with atypically high levels of grooming displacing feeding during the middle part of the test session. Overall, the behavioural profile of AM 251 in a free-feeding context is very similar to (but approximately two-fold less potent than) that recently reported for the parent molecule, rimonabant. Together, these data strongly suggest that the acute anorectic response to CB1 receptor antagonists/inverse agonists is indirectly mediated via major alterations to other components of the behavioural repertoire.

CB1 receptors: emerging evidence for central and peripheral mechanisms that regulate energy balance, metabolism, and cardiovascular health

Insulin resistance, dyslipidaemia and obesity are the major cardiometabolic risk factors contributing to the development of type 2 diabetes and cardiovascular disease (CVD). Owing to the increasing prevalence of obesity, type 2 diabetes, and CVD, new and effective pharmacologic therapies are urgently needed. In this regard, the endogenous cannabinoid system (ECS), a neuromodulatory system involved in the regulation of various aspects of energy balance and eating behaviour through central and peripheral mechanisms, may present the potential to meet this need. In the central nervous system (CNS), cannabinoid type 1 (CB1) receptors and their respective ligands, the endocannabinoids, have a significant role in the modulation of food intake and motivation to consume palatable food. CB1 receptors have also been found in organs involved in the regulation of metabolic homeostasis, such as liver, white adipose tissue, muscle and pancreas. Dysregulation of the ECS has been associated with the development of dyslipidaemia, glucose intolerance, and obesity, and CB1 receptor blockade may have a role in ameliorating these metabolic abnormalities. Thus, pharmacologic options targeting the ECS may provide a novel, effective approach to the prevention and management of CVD, type 2 diabetes and obesity.

Cannabinoids in eating disorders and obesity

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

Cannabinoids excite hypothalamic melanin-concentrating hormone but inhibit hypocretin/orexin neurons: implications for cannabinoid actions on food intake and cognitive arousal

Cannabinoids modulate energy homeostasis and decrease cognitive arousal, possibly by acting on hypothalamic neurons including those that synthesize melanin-concentrating hormone (MCH) or hypocretin/orexin. Using patch-clamp recordings, we compared the actions of cannabinoid agonists and antagonists on identified MCH or hypocretin neurons in green fluorescent protein-expressing transgenic mice. The cannabinoid type-1 receptor (CB1R) agonist R-(+)-[2,3-dihydro-5-methyl-3-(4-morpho linylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone mesylate (WIN55,212,2) depolarized MCH cells and increased spike frequency; in contrast, WIN55,212,2 hyperpolarized and reduced spontaneous firing of the neighboring hypocretin cells, both results consistent with reduced activity seen with intracerebral cannabinoid infusions. These effects were prevented by AM251 [N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide], a CB1R antagonist, and by tetrodotoxin, suggesting no postsynaptic effect on either neuron type. In MCH cells, depolarizing WIN55,212,2 actions were abolished by the GABA(A) receptor antagonist bicuculline, suggesting that the CB1R-mediated depolarization was attributable to reduced synaptic GABA release. WIN55,212,2 decreased spontaneous IPSCs, reduced the frequency but not amplitude of miniature IPSCs, and reduced electrically evoked synaptic currents in MCH cells. Glutamate microdrop experiments suggest that WIN55,212,2 acted on axons arising from lateral hypothalamus local inhibitory cells that innervate MCH neurons. In hypocretin neurons, the reduced spike frequency induced by WIN55,212,2 was attributable to presynaptic attenuation of glutamate release; CB1R agonists depressed spontaneous and evoked glutamatergic currents and reduced the frequency of miniature EPSCs. Cannabinoid actions on hypocretin neurons were abolished by ionotropic glutamate receptor antagonists. Together, these results show that cannabinoids have opposite effects on MCH and hypocretin neurons. These opposing actions could help explain the increase in feeding and reduction in arousal induced by cannabinoids.

Effects of the fatty acid amide hydrolase inhibitor URB597 on the sleep-wake cycle, c-Fos expression and dopamine levels of the rat

Our group has described previously that the endogenous cannabinoid anandamide induces sleep. The hydrolysis of this lipid involves the activity of the fatty acid amide hydrolase (FAAH), which additionally catalyzes the degradation of the satiety factor oleoylethanolamide and the analgesic-inducing lipid palmitoylethanolamide. It has been demonstrated that the inhibition of the FAAH by URB597 increases levels of anandamide, oleoylethanolamide and palmitoylethanolamide in the brain of rats. In order to determinate the physiological properties of the FAAH inhibition on the sleep modulation, we report the pharmacological effects on the sleep-wake cycle of the rat after i.c.v. administrations of URB597, oleoylethanolamide or palmitoylethanolamide (10, 20 microg/5 microl). Separate unilateral i.c.v. injections of 3 compounds during the lights-on period, increased wakefulness and decreased slow wave (SW) sleep in rats in a dose-dependent fashion. We additionally found out that, compared to controls, c-Fos immunoreactivity in hypothalamus and dorsal raphe nucleus was increased in rats that received URB597, oleoylethanolamide or palmitoylethanolamide (10, 20 microg/5 microl, i.c.v.). Next, we found that after an injection of the compounds, levels of dopamine were increased whereas extracellular levels of levodopa (l-DOPA) were decreased. These findings indicate that that inhibition of the FAAH, via URB597, modulates waking. These effects were mimicked separately by the administration of oleoylethanolamide or palmitoylethanolamide. The alertness induced by the compounds tested here activated wake-promoting brain regions and they also induced the release of dopamine. Our results suggest that FAAH activity as well as two molecules that are catalyzed by this enzyme, oleoylethanolamide and palmitoylethanolamide, participate in the regulation of the waking state. Alternative approaches to treat sleep disorders such as excessive somnolence might consider the use of the URB597, oleoylethanolamide or palmitoylethanolamide since all compounds enhance waking.

Cannabinoid CB1 receptor inverse agonists and neutral antagonists: effects on food intake, food-reinforced behavior and food aversions

Drugs that interfere with cannabinoid CB1 receptor transmission suppress a number of food-related behaviors, and these compounds are currently being assessed for their potential utility as appetite suppressants. In addition to rimonabant (SR141716A), several other compounds have been evaluated, including AM251 and AM1387. Biochemical studies indicate that most of the drugs assessed thus far have been CB1 inverse agonists, and these drugs all act to suppress food intake and disrupt food-reinforced behavior. Behavioral tests involving intake of different diets (i.e., high fat, high carbohydrate, laboratory chow) indicate that consumption of all three food types is disrupted by CB1 inverse agonists, and that, expressed as a percent of baseline intake, the effect is roughly comparable across different diets. Although CB1 inverse agonists do not appear to produce severe motor impairments that disrupt feeding behavior, there is evidence that they can induce nausea and malaise. Recent studies have been undertaken to characterize the behavioral effects of CB1 receptor neutral antagonists such as AM4113 to determine if these drugs can reduce feeding and food-reinforced behaviors. Across a variety of different tests, AM4113 produces effects on food-motivated behavior that are very similar to those produced by CB1 inverse agonists. Moreover, this drug did not induce conditioned gaping in rats or vomiting in ferrets. These results suggest that CB1 receptor neutral antagonists may decrease appetite by blocking endogenous cannabinoid tone, and that these drugs may be less associated with nausea than is the case for CB1 inverse agonists.

The endocannabinoid system as an emerging target of pharmacotherapy

The recent identification of cannabinoid receptors and their endogenous lipid ligands has triggered an exponential growth of studies exploring the endocannabinoid system and its regulatory functions in health and disease. Such studies have been greatly facilitated by the introduction of selective cannabinoid receptor antagonists and inhibitors of endocannabinoid metabolism and transport, as well as mice deficient in cannabinoid receptors or the endocannabinoid-degrading enzyme fatty acid amidohydrolase. In the past decade, the endocannabinoid system has been implicated in a growing number of physiological functions, both in the central and peripheral nervous systems and in peripheral organs. More importantly, modulating the activity of the endocannabinoid system turned out to hold therapeutic promise in a wide range of disparate diseases and pathological conditions, ranging from mood and anxiety disorders, movement disorders such as Parkinson's and Huntington's disease, neuropathic pain, multiple sclerosis and spinal cord injury, to cancer, atherosclerosis, myocardial infarction, stroke, hypertension, glaucoma, obesity/metabolic syndrome, and osteoporosis, to name just a few. An impediment to the development of cannabinoid medications has been the socially unacceptable psychoactive properties of plant-derived or synthetic agonists, mediated by CB(1) receptors. However, this problem does not arise when the therapeutic aim is achieved by treatment with a CB(1) receptor antagonist, such as in obesity, and may also be absent when the action of endocannabinoids is enhanced indirectly through blocking their metabolism or transport. The use of selective CB(2) receptor agonists, which lack psychoactive properties, could represent another promising avenue for certain conditions. The abuse potential of plant-derived cannabinoids may also be limited through the use of preparations with controlled composition and the careful selection of dose and route of administration. The growing number of preclinical studies and clinical trials with compounds that modulate the endocannabinoid system will probably result in novel therapeutic approaches in a number of diseases for which current treatments do not fully address the patients' need. Here, we provide a comprehensive overview on the current state of knowledge of the endocannabinoid system as a target of pharmacotherapy.

The role of the endocannabinoid system in the control of energy homeostasis

The endocannabinoid system has recently emerged as an important regulator of energy homeostasis, involved in the control of both appetite and peripheral fat metabolism. We briefly review current understanding of the possible sites of action and cellular mechanisms involved in the central appetitive and peripheral metabolic effects of endocannabinoids. Studies in our laboratory, using leptin-deficient obese rodents and CB1 cannabinoid receptor (CB1)-deficient mice, have indicated that endocannabinoids acting via CB1 are involved in the hunger-induced increase in food intake and are negatively regulated by leptin in brain areas involved in appetite control, including the hypothalamus, limbic forebrain and amygdala. CB1-/- mice are lean and are resistant to diet-induced obesity (DIO) despite similar energy intake to wild-type mice with DIO, suggesting that CB1 regulation of body weight involves additional peripheral targets. Such targets appear to include both adipose tissue and the liver. CB1 expressed in adipocytes has been implicated in the control of adiponectin secretion and lipoprotein lipase activity. Recent findings indicate that both endocannabinoids and CB1 are present in the liver and are upregulated in DIO. CB1 stimulation increases de novo hepatic lipogenesis through activation of the fatty acid biosynthetic pathway. Components of this pathway are also expressed in the hypothalamus where they have been implicated in the regulation of appetite. The fatty acid biosynthetic pathway may thus represent a common molecular target for the central appetitive and peripheral metabolic effects of endocannabinoids.

Endocannabinoids, feeding and suckling--from our perspective

In this overview we have summarized some aspects of our published work related to the effects of the endocannabinoid system on appetite and suckling. As noted also by several other groups we have found that anandamide, a major endocannabinoid, enhances appetite in mice. On partial or full food deprivation over 24 h the levels of 2-arachidonoyl glycerol (2-AG), a second major cannabinoid, are initially elevated in mouse brain; however, partial food deprivation over a longer period causes reduction of 2-AG levels. Blocking the endocannabinoid system with a CB1 antagonist on the 1st day after birth leads to inhibition of suckling; later administration also affects suckling, but does not fully block it.

Integration of endocannabinoid and leptin signaling in an appetite-related neural circuit

Recently developed therapeutics for obesity, targeted against cannabinoid receptors, result in decreased appetite and sustained weight loss. Prior studies have demonstrated CB1 receptors (CB1Rs) and leptin modulation of cannabinoid synthesis in hypothalamic neurons. Here, we show that depolarization of perifornical lateral hypothalamus (LH) neurons elicits a CB1R-mediated suppression of inhibition in local circuits thought to be involved in appetite and "natural reward." The depolarization-induced decrease in inhibitory tone to LH neurons is blocked by leptin. Leptin inhibits voltage-gated calcium channels in LH neurons via the activation of janus kinase 2 (JAK2) and of mitogen-activated protein kinase (MAPK). Leptin-deficient mice are characterized by both an increase in steady-state voltage-gated calcium currents in LH neurons and a CB1R-mediated depolarization-induced suppression of inhibition that is 6-fold longer than that in littermate controls. Our data provide direct electrophysiological support for the involvement of endocannabinoids and leptin as modulators of hypothalamic circuits underlying motivational aspects of feeding behavior.

The emerging role of the endocannabinoid system in endocrine regulation and energy balance

During the last few years, the endocannabinoid system has emerged as a highly relevant topic in the scientific community. Many different regulatory actions have been attributed to endocannabinoids, and their involvement in several pathophysiological conditions is under intense scrutiny. Cannabinoid receptors, named CB1 receptor and CB2 receptor, first discovered as the molecular targets of the psychotropic component of the plant Cannabis sativa, participate in the physiological modulation of many central and peripheral functions. CB2 receptor is mainly expressed in immune cells, whereas CB1 receptor is the most abundant G protein-coupled receptor expressed in the brain. CB1 receptor is expressed in the hypothalamus and the pituitary gland, and its activation is known to modulate all the endocrine hypothalamic-peripheral endocrine axes. An increasing amount of data highlights the role of the system in the stress response by influencing the hypothalamic-pituitary-adrenal axis and in the control of reproduction by modifying gonadotropin release, fertility, and sexual behavior. The ability of the endocannabinoid system to control appetite, food intake, and energy balance has recently received great attention, particularly in the light of the different modes of action underlying these functions. The endocannabinoid system modulates rewarding properties of food by acting at specific mesolimbic areas in the brain. In the hypothalamus, CB1 receptor and endocannabinoids are integrated components of the networks controlling appetite and food intake. Interestingly, the endocannabinoid system was recently shown to control metabolic functions by acting on peripheral tissues, such as adipocytes, hepatocytes, the gastrointestinal tract, and, possibly, skeletal muscle. The relevance of the system is further strenghtened by the notion that drugs interfering with the activity of the endocannabinoid system are considered as promising candidates for the treatment of various diseases, including obesity.

Hypothalamic regulatory pathways and potential obesity treatment targets

With an ever-growing population of obese people as well as comorbidities associated with obesity, finding effective weight loss strategies is more imperative than ever. One of the challenges in curbing the obesity crisis is designing successful strategies for long-term weight loss and weight-loss maintenance. Currently, weight-loss strategies include promotion of therapeutic lifestyle changes (diet and exercise), pharmacological therapy, and bariatric surgery. This review focuses on several pharmacological targets that activate central nervous system pathways that normally limit food intake and body weight. Though it is likely that no single therapy will prove effective for everyone, this review considers several recent pre-clinical targets, and several compounds that have been in human clinical trials.

Endocannabinoids in the regulation of appetite and body weight

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

Delta9-THC induced hyperphagia and tolerance assessment: interactions between the CB1 receptor agonist delta9-THC and the CB1 receptor antagonist SR-141716 (rimonabant) in rats

This study examined effects of the CB1 receptor antagonist/inverse agonist SR-141716 and the CB1 receptor agonist delta9-tetrahydrocannabinol (delta9-THC) on feeding behavior in male Sprague-Dawley rats. Rats were housed individually with free access to regular pelletized laboratory chow [after a 2 weeks handling phase, animals had access to regular chow for 21 h (Study 1) or 22 h (Study 2); high-fat powder food for 3 h in Study 1 and 2 h in Study 2, respectively], and free access to water. Animals were maintained on a reversed 12-h light/dark cycle (dark beginning at noon). Rats were habituated to this type of feeding and light/dark schedule for 3 weeks until a stable baseline for food intake was achieved. In Study 1, animals were examined after administration of delta9-THC alone (dose range 0.1-1.8 mg/kg), SR-141716 alone (dose range 0.03-0.3 mg/kg), and the two drugs combined; injections were given i.p. at the beginning of the second hour after presenting the high-fat diet and drugs were given twice weekly. There was a dose-related increase in high-fat diet intake, peaking at 0.56-1 mg/kg delta9-THC. SR-141716 alone suppressed the high-fat diet intake below control levels. A combination of 0.3 mg/kg SR-141716 and 0.56 mg/kg delta9-THC counteracted the effects on consumption of either drug alone. In Study 2, experimental rats were treated initially with 0.56 mg/kg delta9-THC for six consecutive days; controls received vehicle. Attenuation of the hyperphagia (high-fat diet) was evident after the second injection. Increasing doses of delta9-THC (1 and 1.8 mg/kg, for two and three consecutive days, respectively) did not reinstate the initial hyperphagia. In conclusion, low-to-moderate doses of delta9-THC produced hyperphagia (to a high-fat food source), which was antagonized by SR-141716. SR-141716 singly suppressed intake of the high-fat diet. Delta9-THC-induced hyperphagia dissipated rapidly upon chronic treatment; however, it is unclear whether this reflects pharmacological tolerance or the emergence of a conditioned taste aversion in Study 2.

Paraventricular hypothalamic CB1 cannabinoid receptors are involved in the feeding stimulatory effects of Δ9-tetrahydrocannabinol

BACKGROUND/AIMS

The paraventricular nucleus of the hypothalamus (PVN) is the target of converging orexigenic and anorexigenic pathways originating from various hypothalamic sites and is, therefore, considered to be the chief site mediating hypothalamic regulation of energy homeostasis. Although a large body of evidence suggests that central CB(1) cannabinoid receptors mediate food intake, it is not clear whether PVN CB(1) receptors are involved in the control of feeding behaviour. The present study therefore examined the effects of intra-PVN administration of Delta(9)-tetrahydrocannabinol (THC) and the cannabinoid receptor antagonist SR 141716 on feeding.

METHODS

After being habituated to the test environment and injection procedure, sated rats were injected with SR 141716 (0.03-3.0 microg, Experiment 1) alone or in combination with THC (5.0 microg, Experiment 2) into the PVN. Food intake and locomotor activity then were recorded for 120 min.

RESULTS

Intra-PVN administration of THC produced a significant increase in food intake that was attenuated by SR 141716. Administration of SR 141716 alone did not affect feeding. Locomotor activity was not significantly affected by any drug treatments, suggesting that effects on feeding were not due to a non-specific reduction in motivated behaviour. These findings suggest an important role for PVN cannabinoid signalling in mediating THC-induced feeding behaviour. These results also demonstrate that the blockade of PVN CB(1) receptors alone is insufficient to reduce baseline feeding behaviour under these conditions.

Motivational effects of cannabinoids and opioids on food reinforcement depend on simultaneous activation of cannabinoid and opioid systems

Strong functional interactions exist between endogenous cannabinoid and opioid systems. Here, we investigated whether cannabinoid-opioid interactions modulate motivational effects of food reinforcement. In rats responding for food under a progressive-ratio schedule, the maximal effort (break point) expended to obtain 45 mg pellets depended on the level of food deprivation, with free-feeding reducing break points and food-deprivation increasing break points. Delta-9-tetrahydrocannabinol (THC; 0.3-5.6 mg/kg intrapeitoneally (i.p.)) and morphine (1-10 mg/kg i.p.) dose-dependently increased break points for food reinforcement, while the cannabinoid CB1 receptor antagonist rimonabant (SR-141716A; 0.3-3 mg/kg i.p.) and the preferential mu-opioid receptor antagonist naloxone (0.3-3 mg/kg i.p.) dose-dependently decreased break points. THC and morphine only increased break points when food was delivered during testing, suggesting that these treatments directly influenced reinforcing effects of food, rather than increasing behavior in a nonspecific manner. Effects of THC were blocked by rimonabant and effects of morphine were blocked by naloxone, demonstrating that THC's effects depended on cannabinoid CB1 receptor activation and morphine's effects depended on opioid-receptor activation. Furthermore, THC's effects were blocked by naloxone and morphine's effects were blocked by rimonabant, demonstrating that mu-opioid receptors were involved in the effects of THC and cannabinoid CB1 receptors were involved in the effects of morphine on food-reinforced behavior. Thus, activation of both endogenous cannabinoid and opioid systems appears to jointly facilitate motivational effects of food measured under progressive-ratio schedules of reinforcement and this facilitatory modulation appears to critically depend on interactions between these two systems. These findings support the proposed therapeutic utility of cannabinoid agonists and antagonists in eating disorders.

Hormonal regulation of food intake

Our knowledge of the physiological systems controlling energy homeostasis has increased dramatically over the last decade. The roles of peripheral signals from adipose tissue, pancreas, and the gastrointestinal tract reflecting short- and long-term nutritional status are now being described. Such signals influence central circuits in the hypothalamus, brain stem, and limbic system to modulate neuropeptide release and hence food intake and energy expenditure. This review discusses the peripheral hormones and central neuronal pathways that contribute to control of appetite.

Endocannabinoids and liver disease--review

AIMS

Endocannabinoids are endogenous compounds that bind to the same receptors as tetrahydrocannabinol, the active component in marijuana and hashish. They have been found to have many physiological and patho-physiological functions, including mood alteration, control of feeding and appetite, motor and co-ordination activities, analgesia, immune modulation and gut motility. In this review we aim to elucidate current knowledge as to their role in liver physiology and disease.

METHODS

The major findings published to date concerning endocannabinoids and liver disease are described, and their implications with regard to understanding disease mechanisms, and the development of new treatments is considered.

RESULTS

Recently, endocannabinoids have been implicated in the hemodynamic alterations occurring in cirrhosis. These changes appear to be mediated via specific cannabinoid receptors (CB1) on splanchnic and hepatic vascular endothelium. Plasma levels of endocannabinoids also seem to be elevated in hepatitis, and are involved in apoptosis of hepatocytes by a membrane mechanism not related to a specific receptor. Other studies suggest a beneficial role for cannabinoids in reducing the inflammation of experimental hepatitis. In an animal model of acute hepatic failure, both endocannabinoids and the antagonist to the CB1 receptor have been found to have a beneficial effect on neurological and cognitive function.

CONCLUSIONS

Endocannabinoids appear to be involved in several aspects of acute and chronic liver disease, including vascular changes, modulation of inflammatory process and neurological function, Further research may provide new insights into the pathophysiology of liver disease, as well as a basis for novel treatment modalities.

Cannabinoids augment the release of neuropeptide Y in the rat hypothalamus

Little is known about the mechanism of action behind the orexigenic activity of cannabinoids. Neuropeptide Y (NPY) is one of the most potent orexigenic factors and is a key mediator in the hypothalamic control of food intake. We examined the effect of cannabinoids on NPY release using a rat hypothalamic explant model. The cannabinoid agonists anandamide (AEA) and CP55,940 both significantly augmented resting and KCl-evoked NPY release. AM251, a cannabinoid receptor antagonist, blocked the augmentation of NPY release elicited by AEA and CP55,940. Additionally, AM251 administered alone, in the absence of exogenous cannabinoid agonists, inhibited NPY release demonstrating the role of endogenous cannabinoids in NPY release. Combined, these findings demonstrate that cannabinoids augment NPY release in the hypothalamus and that this may be a potential mechanism behind the orexigenic activity of cannabinoids.

Differential effects of two cannabinoid receptor agonists on progressive ratio responding for food and free-feeding in rats

The cannabinoid receptor agonists delta9-tetrahydrocannabinol (delta9-THC) and HU-210 were compared in terms of their effects on: (1) progressive ratio (PR) responding for food, and (2) free food intake. In the first experiment, food-deprived Wistar rats were trained on a time-constrained (60 min) PR-5 schedule for food reinforcement, in which the response requirement incremented by five lever presses for each successive reinforcer. One group of rats received vehicle, 0.5, 1 or 3 mg/kg delta9-THC (i.p.), and three other groups received HU-210 (i.p.) at three different dose ranges, spanning 0.001-0.1 mg/kg. In the second experiment, the effects of the two drugs on free food intake were tested in a separate group of non-deprived rats. For PR responding, delta9-THC significantly increased the break point (final ratio completed) and the total number of lever presses emitted. The same drug also significantly increased free food intake. However, the effects of HU-210 were quite different: it did not alter PR responding at any dose; instead, its only significant effect was to reduce free food intake at 0.06 mg/kg. These data suggest that increased motivation to obtain food might underlie the hyperphagic effects of delta9-THC. However, the synthetic agonist HU-210 has different effects: it only acts to reduce feeding behaviour, an outcome that probably reflects non-specific behavioural disruption. These findings suggest important differences between the two CB1 receptor agonists in terms of their pharmacological effects.

The endocannabinoid system in the brain of Carassius auratus and its possible role in the control of food intake

Cannabinoid receptors and the endocannabinoids anandamide and 2-arachidonoylglycerol have been suggested to regulate food intake in several animal phyla. Orthologs of the mammalian cannabinoid CB(1) and CB(2) receptors have been identified in fish. We investigated the presence of this endocannabinoid system in the brain of the goldfish Carassius auratus and its role in food consumption. CB(1)-like immunoreactivity was distributed throughout the goldfish brain. The prosencephalon showed strong CB(1)-like immunoreactivity in the telencephalon and the inferior lobes of the posterior hypothalamus. Endocannabinoids were detected in all brain regions of C. auratus and an anandamide-hydrolysing enzymatic activity with features similar to those of mammalian fatty acid amide hydrolase was found. Food deprivation for 24 h was accompanied by a significant increase of anandamide, but not 2-arachidonoylglycerol, levels only in the telencephalon. Anandamide caused a dose-dependent effect on food intake within 2 h of intraperitoneal administration to satiated fish and significantly enhanced or reduced food intake at low (1 pg/g body weight) or intermediate (10 pg/g) doses, respectively, the highest dose tested (100 pg/g) being inactive. We suggest that endocannabinoids might variously contribute to adaptive responses to food shortage in fish.

Endocannabinoid control of food intake and energy balance

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

Endocannabinoids and food consumption: comparisons with benzodiazepine and opioid palatability-dependent appetite

The endocannabinoid system consists of several endogenous lipids, including anandamide and 2-arachidonoyl-glycerol (2-AG), and constitute a retrograde signalling system, which modulates neurotransmitter release and synaptic plasticity. Specific brain-type cannabinoid receptors (CB(1)) are widely distributed in the central nervous system, and are localized presynaptically. Mounting evidence, reviewed here, indicates that cannabinoids can act to increase food consumption, and cannabinoid CB(1) receptor antagonists/inverse agonists reduce food intake and suppress operant responding for food rewards. Hence, endocannabinoids provide the first example of a retrograde signalling system, which is strongly implicated in the control of food intake. Benzodiazepine and opioid palatability-dependent appetite are well-established processes supported by several sources of convergent evidence; they provide pharmacological benchmarks against which to evaluate the endocannabinoids. To date, evidence that endocannabinoids specifically modulate palatability as an affective evaluative process is insufficient and not compelling. Endocannabinoids may have important clinical utility in the treatment of human obesity and forms of eating disorders.

Blood levels of the endocannabinoid anandamide are increased in anorexia nervosa and in binge-eating disorder, but not in bulimia nervosa

The endocannabinoid system, consisting of two cannabinoid receptors (CB1 and CB2) and the endogenous ligands anandamide (arachidonoylethanolamide (AEA)) and 2-arachidonoylglycerol (2-AG), has been shown to control food intake in both animals and humans, modulating either rewarding or quantitative aspects of the eating behavior. Moreover, hypothalamic endocannabinoids seem to be part of neural circuitry involved in the modulating effects of leptin on energy homeostasis. Therefore, alterations of the endocannabinoid system could be involved in the pathophysiology of eating disorders, where a deranged leptin signalling has been also reported. In order to verify this hypothesis, we measured plasma levels of AEA, 2-AG, and leptin in 15 women with anorexia nervosa (AN), 12 women with bulimia nervosa (BN), 11 women with binge-eating disorder (BED), and 15 healthy women. Plasma levels of AEA resulted significantly enhanced in both anorexic and BED women, but not in bulimic patients. No significant change occurred in the plasma levels of 2-AG in all the patients' groups. Moreover, circulating AEA levels were significantly and inversely correlated with plasma leptin concentrations in both healthy controls and anorexic women. These findings show for the first time a derangement in the production of the endogenous cannabinoid AEA in drug-free symptomatic women with AN or with BED. Although the pathophysiological significance of this alteration awaits further studies to be clarified, it suggests a possible involvement of AEA in the mediation of the rewarding aspects of the aberrant eating behaviors occurring in AN and BED.

Effects of the endocannabinoid noladin ether on body weight, food consumption, locomotor activity, and cognitive index in mice

We have investigated the effect of 2-arachidonylglyceryl-ether (Noladin) on food consumption, weight, activity, and cognitive function in mice during diet restriction for 17 days and subsequent ad libitum feeding for 32 days. Female Sabra mice were given food for 2.5 h/day (equal to 60% diet restriction), received Noladin (0.001, 0.01, 0.1 mg/(kg day) intraperitonially (i.p.)) with or without the CB1 antagonist SR141716A (1 mg/kg i.p.) during days 3-17. Noladin (0.001 mg/kg) significantly increased food consumption without a change in body weight, probably due to increased activity and there was no change in cognitive function. A higher dose (0.1 mg/kg) did not affect food consumption, but increased activity and slightly decreased weight 32 days after termination of Noladin administration; however, cognitive deterioration was observed. At all doses tested, Noladin did not affect weight during the diet-restriction period, whereas the CB1 antagonist (with or without Noladin) caused a very significant decline in weight in this phase. Weight catch-up was observed 1 month after administration of Noladin was discontinued. Weight at day 32 after the termination of Noladin (0.1 mg/(kg day)) treatment was 5% less than control. Female C57BL/6 mice (same protocol, with 0.001 mg/(kg day) Noladin) gave similar results to 0.1 mg/kg in Sabra mice as regards weight. CB1 antagonist treatment caused very significant decline in both weight and food consumption; cognition and activity were unchanged. These results indicate that Noladin has a significant dose-dependent effect on food consumption, cognition and weight maintenance after weight loss. Low doses of Noladin may possibly allow an increase in food intake without a gain in weight after dieting. Thus, Noladin could be of potential clinical benefit in treating disorders of body weight. Noladin seems to signal food consumption and weight through CB1 receptors based on effects observed with the CB1 antagonist, while the cognition and activity are probably mediated by non-cannabinoid receptors.

Cocaine- and amphetamine-related transcript is involved in the orexigenic effect of endogenous anandamide

Endocannabinoids acting at CB1 cannabinoid receptors (CB1) increase appetite. In view of the predominant presynaptic localization of CB1 in the brain, we tested the hypothesis that the orexigenic effect of endocannabinoids involves inhibition of the release of a tonically active anorexigenic mediator, such as the peptide product of the cocaine- and amphetamine-related transcript (CART). The CB1 antagonist rimonabant inhibited food intake in food-restricted wild-type mice, but not in their CART-deficient littermates. Mice deficient in fatty acid amide hydrolase (FAAH), the enzyme responsible for the in vivo metabolism of the endocannabinoid anandamide, have reduced levels of CART-immunoreactive nerve fibers and terminals in several brain regions implicated in appetite control, including the arcuate, dorsomedial and periventricular nuclei of the hypothalamus, the amygdala, the bed nucleus of the stria terminalis and the nucleus accumbens, and treatment of FAAH(-/-) mice with rimonabant, 3 mg/kg/day for 7 days, increased CART levels toward those seen in FAAH(+/+) wild-type controls. In contrast, no difference in the density of CART-immunoreactive fibers was observed in the median eminence and the paraventricular nucleus of FAAH(+/+) and FAAH(-/-) mice. Acute treatment of wild-type mice with the cannabinoid agonist HU-210 resulted in elevated CART levels in the dorsomedial nucleus and the shell portion of the nucleus accumbens. These observations are compatible with CART being a downstream mediator of the CB1-mediated orexigenic effect of endogenous anandamide.

Emerging aspects of pharmacotherapy for obesity and metabolic syndrome

Obesity is a multifactorial, chronic disorder that has reached epidemic proportions in most industrialized countries and is threatening to become a global epidemic. Obese patients are at higher risk from coronary artery disease, hypertension, hyperlipidemia, diabetes mellitus, cancers, cerebrovascular accidents, osteoarthritis, restrictive pulmonary disease, and sleep apnoea. In particular, visceral fat accumulation is usually accompanied by insulin resistance or type 2 diabetes mellitus, hypertension, hypertriglyceridemia, high uremic acid levels, low high density lipoprotein (HDL) cholesterol to define a variously named syndrome or metabolic syndrome. Metabolic syndrome is now considered a major cardiovascular risk factor in a large percentage of population in worldwide. Both obesity and metabolic syndrome are particularly challenging clinical conditions to treat because of their complex pathophysiological basis. Indeed, body weight represents the integration of many biological and environmental components and relationships among fat and glucose tolerance or blood pressure are not completely understood. Efforts to develop innovative anti-obesity drugs, with benefits for metabolic syndrome, have been recently intensified. In general two distinct strategies can be adopted: first, to reduce energy intake; second, to increase energy expenditure. Here we review some among the most promising avenues in these two fields of drug therapy of obesity and, consequently, of metabolic syndrome.

Arachidonylethanolamide Induces Apoptosis of Human Glioma Cells through Vanilloid Receptor-1

The anti-tumor properties of cannabinoids have recently been evidenced, mainly with delta9-tetrahydrocannabinol (THC). However, the clinical application of this drug is limited by possible undesirable side effects due to a broad expression of cannabinoid receptors (CB1 and CB2). An attractive field of research therefore is to identify molecules with more selective tumor targeting. This is particularly important for malignant gliomas, considering their poor prognosis and their location in the brain. Here we investigated whether the most potent endogenous cannabinoid, arachidonylethanolamide (AEA), could be a candidate. We observed that AEA induced apoptosis in long-term and recently established glioma cell lines via aberrantly expressed vanilloid receptor-1 (VR1). In contrast with their role in THC-mediated death, both CB1 and CB2 partially protected glioma against AEA-induced apoptosis. These data show that the selective targeting of VR1 by AEA or more stable analogues is an attractive research area for the treatment of glioma.

Oleamide and anandamide effects on food intake and sexual behavior of rats

Oleamide is a lipid with diverse properties, including cannabinoid-like activity. For example, it induces the classic triad of effects attributable to these molecules: decrease in core temperature, hypolocomotion, and reduction in pain perception. However, as it binds to the cannabinoid receptors (CB1) only at high concentrations, it is not considered an actual endocannabinoid. In this study, we tested the effect of oleamide on food intake and sexual behavior and compared it to the effect induced by anandamide. Results indicate that oleamide and anandamide increased food intake during the 3h post-injection. In addition, anandamide but not oleamide induced changes in sexual performance. This study further supports the role of endocannabinoids in food ingestion and male sexual behavior and gives additional support to the notion that, although oleamide might not be an endocannabinoid, it shares some effects with them.

The inhibitory effect of anandamide on luteinizing hormone-releasing hormone secretion is reversed by estrogen

Because Delta-9-tetrahydrocannabinol (THC) inhibited luteinizing hormone-releasing hormone (LHRH) in male rats, we hypothesized that the endocannabinoid, anandamide (AEA), would act similarly. AEA microinjected intracerebroventricularly (i.c.v.) decreased plasma luteinizing hormone (LH) at 30 min in comparison to values in controls (P < 0.001). The cannabinoid receptor 1 (CB1-r)-specific antagonist, [N-(piperidin-1-yl)-1-(2,4-dichlorophenyl)-5-(4-chlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide] (AM251), produced a significant elevation in plasma LH (P < 0.01). AEA (10(-9) M) decreased LHRH release from medial basal hypothalami incubated in vitro. These results support the concept that endogenous AEA inhibits LHRH followed by decreased LH release in male rats. In ovariectomized (OVX) female rats, AEA i.c.v. also inhibited LH release, but in this case AM251 had an even greater inhibitory effect than AEA. In vitro, AEA had no effect on LHRH in OVX rats. It seems that endogenous AEA inhibits LHRH followed by decreased LH release in OVX rats but that AM251 has an inhibitory action in this case. In striking contrast, in OVX, estrogen-primed (OVX-E) rats, AEA i.c.v. instead of decreasing LH, increased its release. This effect was completely blocked by previous injection of AM251. When medial basal hypothalami of OVX-E rats were incubated, AEA increased LHRH release. The synthesized AEA was higher in OVX-E rats than in OVX and males, indicating that estrogen modifies endocannabinoid levels and effects. The results are interpreted to mean that sex steroids have profound effects to modify the response to AEA. It inhibits LHRH and consequently diminishes LH release in males and OVX females, but stimulates LHRH followed by increased LH release in OVX-E-primed rats.

Very low doses of Δ8-THC increase food consumption and alter neurotransmitter levels following weight loss

We have investigated the effect of 0.001 mg/kg delta(8)-tetrahydrocannabinol (THC) on food consumption, cognitive function, and neurotransmitters in mice. Sabra mice were treated with vehicle, THC, or THC+CB1 antagonist (SR141716A). The mice were fed for 2.5 h a day for 9 or 50 days. In the 9-day schedule, THC-treated mice showed a 16% increase in food intake compared with controls (P<.001). This effect was reversed by the antagonist (P<.01). In the long-term schedule a 22% increase in intake (P<.05) was recorded. During the course of the 9- and 50-day experimental protocol, all mice lost about 20% and 10% of their original weight, respectively, to reach approximately the same weights, which were not significantly different between the different treatment groups. In addition, THC caused an increase in activity (P<.05). Cognitive function showed a tendency to improve (P<.06) in the THC-treated mice, which was reversed by the antagonist for Days 4 and 5 of the maze (P<.01, and P<.05, respectively). Significant decreases in dopamine and serotonin (5-HT) levels were found both in the hypothalamus (P<.01) and the hippocampus (P<.01, P<.05), respectively, while norepinephrine (NE) levels showed tendency to increase in both the hypothalamus and hippocampus. Delta(8)-THC increased food intake significantly more (P<.05) than did delta(9)-THC, while performance and activity were similar. Thus, delta(8)-THC (0.001 mg/kg) caused increased food consumption and tendency to improve cognitive function, without cannabimimetic side effects. Hence, a low dose of THC might be a potential therapeutic agent in the treatment of weight disorders.

Association study of cannabinoid receptor gene (CNR1) alleles and anorexia nervosa: differences between restricting and binging/purging subtypes

Anorexia nervosa (AN) is a severe and disabling psychiatric disorder, characterized by profound weight loss and body image disturbance. Family and twin studies indicate a significant genetic contribution to this disorder although no genetic mutation has yet been identified. The endocannabinoid system has recently been implicated in many physiological functions including appetite regulation. We, therefore, undertook a family based study to test the hypothesis whether a polymorphism of the CNR1 gene, which encodes human CB1 receptor, a subclass of the central cannabinoid receptor, contributes to the susceptibility to AN. Fifty two families (parents with one or two affected siblings) were genotyped for the (AAT) trinucleotide repeat of CNR1 gene. Using the haplotype relative risk (HRR) method, the distribution of alleles transmitted to the patients was not found to be significantly different from the non-transmitted parental alleles. However, upon dividing the samples to restricting and binging/purging subtypes of AN, the extended transmission disequilibrium test (ETDT) revealed that there is preferential transmission of different alleles in each of the subtypes. The 14 repeat allele was preferentially transmitted in the binging/purging AN group (P = 0.05) but not in the restricting AN group, whereas the 13 repeat allele was preferentially transmitted in the restricting AN group (almost significant, P = 0.07) but not in the binging/purging AN group. Our study suggests that restricting AN and binging/purging AN may be associated with different alleles of the CNR1 gene.

Endocannabinoid Receptor Antagonists

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.

Circuitries Involved in the Control of Energy Homeostasis and the Hypothalamic-Pituitary-Adrenal Axis Activity

The regulation of bodyweight is a complex process involving the interplay of neuronal circuitries controlling food intake and energy expenditure (thermogenesis) with endocrine secretions modulating the activity of the neurons making up those circuitries. The neurons controlling food intake and thermogenesis also modulate the hypothalamic-pituitary-adrenal axis, the role of which in the regulation of energy balance has been acknowledged for some time. These neurons secrete various neuromolecules or neuropeptides including endocannabinoids, neuropeptide Y, agouti-related protein, melanin-concentrating hormone, orexins (hypocretins), melanocortins, cocaine- and amphetamine-regulated transcript, thyrotropin-releasing hormone, corticotropin-releasing hormone, and urocortins. Among those peptides, neuropeptide Y, agouti-related peptide, melanin-concentrating hormone, orexins, and endocannabinoids have been classified as being anabolic molecules whereas melanocortins, cocaine- and amphetamine-regulated transcript, thyrotropin-releasing hormone, and corticotropin-releasing hormone are referred to as catabolic peptides. The expression and secretion of these neuromolecules are known to be affected by the anabolic (corticosteroids and ghrelin) and catabolic (leptin, insulin, and glucagon-like peptide 1) peripheral hormones. A link is made between the pathways regulating energy balance and those modulating the activity of the hypothalamic-pituitary-adrenal axis.

The cannabinoid CB1 antagonists SR 141716A and AM 251 suppress food intake and food-reinforced behavior in a variety of tasks in rats

Cannabinoid CB1 receptor agonists, including delta-9-tetrahydrocannabinol (Delta 9-THC) (the main psychoactive ingredient in marijuana) have been shown to increase feeding in rats and humans. Conversely, it has been reported that acute administration of the CB1 receptor antagonist SR 141716A reduces food intake in rats. Based upon this observation, it has been suggested that CB1 antagonists could be useful as appetite suppressant drugs. The present studies were designed to provide a detailed examination of the effects of CB1 antagonists on food intake across a range of paradigms. Two CB1 antagonists (SR 141716A and AM 251) were administered to rats trained on fixed-ratio schedules with two different ratio requirements (fixed-ratio 1 and fixed-ratio 5). Both drugs produced a dose-dependent decrease in lever pressing, and had a relatively long duration of action (T1/2: SR 141716A, 15.1 h; AM 251, 22.0 h). Furthermore, intake of three diets with differing macronutrient composition (lab chow, high fat, high carbohydrate) was studied. Both drugs significantly suppressed intake of all three foods, and there were no significant interactions between drug dose and diet type. These findings support the hypothesis that CB1 receptor antagonists could be useful pharmacological tools for the suppression of appetite.

Biphasic effects of cannabinoids on acetylcholine release in the hippocampus: site and mechanism of action

Cannabinoids have been shown to critically modulate cholinergic neurotransmission in the hippocampus, yet opposing effects of cannabinoid receptor 1 (CB1R) agonists on hippocampal synaptic acetylcholine (ACh) efflux have been reported. This study shows that administration of a synthetic CB1R agonist results in a biphasic, dose-dependent, effect on hippocampal ACh: a low (0.5 mg/kg, i.p.) and a high (5 mg/kg, i.p) dose of WIN55,212-2 induces a transient stimulation and a prolonged inhibition of hippocampal ACh efflux, respectively. Both effects of WIN55,212-2 are mediated through CB1 receptors coupled to Gi but involve different neuroanatomical sites. Thus, intrahippocampal infusion of the CB1R antagonist SR141716A or pertussis toxin blocked the inhibition of hippocampal ACh release induced by the high dose of WIN55,212-2, but was without effect on the stimulatory action of the low dose. In contrast, this latter effect was blocked by SR141716A or pertussis toxin infused, in dual microdialysis experiments, in the septum, in which the majority of cholinergic cell bodies projecting to the hippocampus reside. The stimulatory and inhibitory effects of WIN55,212-2 on hippocampal ACh involve dopamine D1 and D2 receptor activation, respectively, given that pretreatment with D1 and D2 receptor antagonists prevents the respective actions of WIN55,212-2. We propose that the in vivo observed biphasic effects of CB1R agonists on hippocampal ACh release result from a differential, functional association of anatomicaly distinct subpopulations of CB1-Gi coupled receptors to neurotransmitter systems that have opposing effects on ACh release. This concept could provide a theoretical framework to understand endocannabinoids as state-dependent modulators of neuronal activity.

The endocannabinoid system in the basal ganglia and in the mesolimbic reward system: implications for neurological and psychiatric disorders

To date, N-arachidonoylethanolamine (anandamide) and 2-arachidonoylglycerol are the best studied endocannabinoids and are thought to act as retrograde messengers in the central nervous system (CNS). By activating presynaptic cannabinoid CB1 receptors, they can reduce glutamate release in dorsal and ventral striatum (nucleus accumbens) and alter synaptic plasticity, thereby modulating neurotransmission in the basal ganglia and in the mesolimbic reward system. In this review, we will focus on the role of the endocannabinoid system within these neuronal pathways and describe its effect on dopaminergic transmission and vice versa. The endocannabinoid system is unlikely to directly affect dopamine release, but can modify dopamine transmission trough trans-synaptic mechanisms, involving gamma-aminobutyric acid (GABA)-ergic and glutamatergic synapses, as well as by converging signal transduction cascades of the cannabinoid and dopamine receptors. The dopamine and endocannabinoid systems exert a mutual control on each other. Cannabinergic signalling may lead to release of dopamine, which can act via dopamine D1-like receptors as a negative feedback mechanism to counteract the effects of activation of the cannabinoid CB1 receptor. On the other hand, dopaminergic signalling via dopamine D2-like receptors may lead to up-regulation of cannabinergic signalling, which is likely to represent a negative feedback on dopaminergic signalling. The consequences of these interactions become evident in pathological conditions in which one of the two systems is likely to be malfunctioning. We will discuss neurological and psychiatric disorders such as Parkinson's and Huntington's disease, drug addiction and schizophrenia. Furthermore, the possible role of the endocannabinoid system in disorders not necessarily depending on the dopaminergic system, such as eating disorders and anxiety, will be described.

The cannabinoid system: a role in both the homeostatic and hedonic control of eating?

Knowledge of the cannabinoid system and its components has expanded greatly over the past decade. There is increasing evidence for its role in the regulation of food intake and appetite. Cannabinoid system activity in the hypothalamus is thought to contribute to the homeostatic regulation of energy balance, under the control of the hormone leptin. A second component of cannabinoid-mediated food intake appears to involve reward pathways and the hedonic aspect of eating. With the cannabinoid system contributing to both regulatory pathways, it presents an attractive therapeutic target for the treatment of both obesity and eating disorders.

Short-term fasting and prolonged semistarvation have opposite effects on 2-AG levels in mouse brain

2-Arachidonoyl glycerol (2-AG) levels in whole mouse brain and two of its regions-hippocampus and hypothalamus-were determined after diet restriction (between 60 and 40%) lasting 12 days. The diet restriction lowered the level of 2-AG, which in the hypothalamus depended on the severity of the diet restriction, while the level in the hippocampus was not dependent on the diet regimen. As these observations differ from previously published data showing elevation of 2-AG levels in rat brain after 24 h of severe food restriction, we measured 2-AG levels in whole mouse brain after a comparable period of full starvation (fasting). We confirmed the elevation of 2-AG levels. It seems possible that these time-dependent variations of 2-AG levels may be of importance as a general coping strategy by animals during periods of starvation.

The endogenous cannabinoid system affects energy balance via central orexigenic drive and peripheral lipogenesis

The cannabinoid receptor type 1 (CB1) and its endogenous ligands, the endocannabinoids, are involved in the regulation of food intake. Here we show that the lack of CB1 in mice with a disrupted CB1 gene causes hypophagia and leanness. As compared with WT (CB1+/+) littermates, mice lacking CB1 (CB1-/-) exhibited reduced spontaneous caloric intake and, as a consequence of reduced total fat mass, decreased body weight. In young CB1-/- mice, the lean phenotype is predominantly caused by decreased caloric intake, whereas in adult CB1-/- mice, metabolic factors appear to contribute to the lean phenotype. No significant differences between genotypes were detected regarding locomotor activity, body temperature, or energy expenditure. Hypothalamic CB1 mRNA was found to be coexpressed with neuropeptides known to modulate food intake, such as corticotropin-releasing hormone (CRH), cocaine-amphetamine-regulated transcript (CART), melanin-concentrating hormone (MCH), and preproorexin, indicating a possible role for endocannabinoid receptors within central networks governing appetite. CB1-/- mice showed significantly increased CRH mRNA levels in the paraventricular nucleus and reduced CART mRNA levels in the dorsomedial and lateral hypothalamic areas. CB1 was also detected in epidydimal mouse adipocytes, and CB1-specific activation enhanced lipogenesis in primary adipocyte cultures. Our results indicate that the cannabinoid system is an essential endogenous regulator of energy homeostasis via central orexigenic as well as peripheral lipogenic mechanisms and might therefore represent a promising target to treat diseases characterized by impaired energy balance.

The cannabinoid receptor antagonist SR 141716 attenuates overfeeding induced by systemic or intracranial morphine

RATIONALE

Considerable interplay exists between the brain's opioid and cannabinoid systems. These systems are both involved in the control of appetite and research supports the notion that the opioid system modulates the role of the cannabinoid system on appetite. However, the ability of the cannabinoid system to modulate the opioid system's control over appetite has not been well studied.

OBJECTIVES

The present study examined the role of cannabinoid CB(1) receptors in the control of opioid-induced feeding, and sought to identify specific brain regions underlying this role.

METHODS

After being habituated to the test environment and injection procedure, sated rats were injected with the cannabinoid CB(1) receptor antagonist SR 141716 (0.03-3.0 mg/kg, IP). Thirty minutes later, morphine or its vehicle were administered systemically (2.5 mg/kg SC, experiments 1 and 2) or intracranially into the nucleus accumbens (nAcc, experiment 3) or paraventricular nucleus of the hypothalamus (PVN, experiment 4). Food intake and locomotor activity was then recorded for 120 min.

RESULTS

A significant increase in food intake was observed following systemic and intracranial (10 nmol) application of morphine in all experiments. SR 141716 suppressed systemic and intra-PVN morphine induced feeding (experiments 2 and 4), but did not attenuate food intake induced by intra-nAcc application of morphine (experiment 3).

CONCLUSIONS

Because SR 141716 had no effect on intra-nAcc morphine-stimulated feeding, it would appear that cannabinoid receptors do not modify opioid-mediated hedonic responses to food. Rather, we conclude that cannabinoid CB(1) receptor blockade may suppress opioid-induced feeding by stimulating the release of satiety-related peptides within the hypothalamus. Further, because SR 141716 did not block morphine induced locomotor activity, the observed effects on feeding do not appear to be due to a non-specific reduction in motivated behaviour.

The differential effects of food restriction on 5-HT1A and 5-HT1B receptor mediated control of serotonergic transmission in the hippocampus and hypothalamus of rats

Serotonergic pathways are considered important in the regulation of appetite. We have determined, in female rats, the effects of 4 weeks food restriction (FR) on serotonin function, using in vivo microdialysis. We recorded basal 5-HT release in the hypothalamus and hippocampus, and the sensitivity of the somatodendritic 5-HT1A autoreceptors in the raphe nuclei, and the nerve terminal 5-HT1B autoreceptors which together regulate the synthesis and release of 5-HT in these regions. Sensitivity of the somatodendritic 5-HT1A autoreceptors was assessed by measuring the reduction in extracellular 5-HT induced by systemic administration of the 5-HT1A receptor agonist 8-hydroxy-2-di-n-(propylamino)-tetralin (8-OH-DPAT), while sensitivity of nerve terminal 5-HT1B autoreceptors was measured by observing the increase in 5-HT release after systemic injection of the 5-HT1B receptor antagonist GR 127935. Basal release of 5-HT was not affected by FR. 8-OH-DPAT decreased 5-HT release in the hippocampus and hypothalamus in both groups, while GR 127935 increased 5-HT release in both areas in the control animals but not in the hypothalamus of the FR animals. Since 5-HT1B receptors regulate 5-HT release by a negative feedback mechanism, the decrease in sensitivity of 5-HT1B receptors in the hypothalamus of FR rats indicates increased serotonergic transmission in these rats. The fact that such differential effects on 5-HT release appeared only in the hypothalamus, the center of regulation of energy balance, suggests a compensatory role in FR by increasing 5-HT secretion, thereby reducing feeding behavior.

Does short-term diet restriction in mice precipitate the development of anorexia?

Anorexia nervosa (AN) inevitably begins with dieting. Yet, it is unknown whether anyone who will ultimately suffer from anorexia is already ill upon "going on a diet", or whether disease begins during, and is perhaps triggered by, dieting. The objective of the following study was to precipitate anorexia by imposing diet restriction on animals, as a model for generating AN in humans. Three hundred young female Sabra mice were diet restricted to 40% of daily nutrient requirements for 12 days, lost 17% of body weight and were then re-fed ad-lib. All mice regained appetite and weight. Our conclusions are that diet restriction does not precipitate anorexia in mice. Our findings do not support a role for diet restriction per se in triggering AN.

Animal models in the investigation of anorexia

Anorexia nervosa (AN) is an eating disorder of unknown origin that most commonly occurs in women and usually has its onset in adolescence. Patients with AN invariably have a disturbed body image and an intense fear of weight gain. There is currently no definitive treatment for this disease, which carries a 20% mortality over 20 years. Development of an appropriate animal model of AN has been difficult, as the etiology of this eating disorder likely involves a complex interaction between genetic, environmental, social, and cultural factors. In this review, we focus on several possible rodent models of AN. In our laboratory, we have developed and studied three different mouse models of AN based on clinical profiles of the disease; separation stress, activity, and diet restriction (DR). In addition, we discuss the spontaneous mouse mutation anx/anx and several mouse gene knockout models, which have resulted in an anorexic phenotype. We highlight what has been learned from each of these models and possibilities for future models. It is hoped that a combination of the study of such models, together with genetic and clinical studies in patients, will lead to more rational and successful prevention/treatment of this tragic, and often fatal, disease.

Water maze performance and changes in serum corticosterone levels in zinc-deprived and pair-fed rats

The aims of the present study were (1) to evaluate the learning and short- and long-term memory of zinc-deprived (ZD) and pair-fed (PF) rats in a Morris water maze (MWM) and (2) to monitor the serum corticosterone levels of these rats before and after swimming. Young Sprague-Dawley rats (aged 27-31 days) consumed AIN-93G diet for 10 days, and then were separated into ad libitum control (CT), PF and ZD groups. The zinc content of the diet was 25-30 ppm (CT and PF) or <1 ppm (ZD). After 17 days on experimental diets, a MWM was used to test spatial cognition. Delayed-matching-to-place (DMP) test results indicate that both zinc deprivation and food restriction had no effect on short-term memory. The PF rats exhibited significantly impaired learning and thigmotaxia (i.e., wall hugging) in the learning test. The PF group also demonstrated less preference for the target zone in the first 15 s of the probing test. When the total 120 s of the probing test was considered, there were no differences in preference for the target zone, but thigmotaxia was greater in the PF than the CT group. The only behavioral change of the ZD group was thigmotaxia observed during the 120-s probing test following training, indicating the increment of anxiety. Morning basal corticosterone levels before swim training were significantly elevated in the PF group on Day 15 of dietary treatment, whereas a significant elevation of the basal corticosterone level in the ZD group was not statistically significant until Day 22. The data indicate an association between impaired learning, poor searching strategy and elevated corticosterone in the PF group. In contrast, the ZD rats showed normal cognitive performance but had elevated corticosterone and increased anxiety-like behavior (thigmotaxia).

Endogenous cannabinoid system as a modulator of food intake

The ability of Cannabis sativa (marijuana) to increase hunger has been noticed for centuries, although intensive research on its molecular mode of action started only after the characterization of its main psychoactive component Delta(9)-tetrahydrocannabinol in the late 1960s. Despite the public concern related to the abuse of marijuana and its derivatives, scientific studies have pointed to the therapeutic potentials of cannabinoid compounds and have highlighted their ability to stimulate appetite, especially for sweet and palatable food. Later, the discovery of specific receptors and their endogenous ligands (endocannabinoids) suggested the existence of an endogenous cannabinoid system, providing a physiological basis for biological effects induced by marijuana and other cannabinoids. Epidemiological reports describing the appetite-stimulating properties of cannabinoids and the recent insights into the molecular mechanisms underlying cannabinoid action have proposed a central role of the cannabinoid system in obesity. The aim of this review is to provide an extensive overview on the role of this neuromodulatory system in feeding behavior by summarizing the most relevant data obtained from human and animal studies and by elucidating the interactions of the cannabinoid system with the most important neuronal networks and metabolic pathways involved in the control of food intake. Finally, a critical evaluation of future potential therapeutical applications of cannabinoid antagonists in the therapy of obesity and eating disorders will be discussed.