Leptin-regulated endocannabinoids are involved in maintaining food intake

The endocannabinoid system: function in survival of the embryo, the newborn and the neuron

Since the identification and cloning of the first cannabinoid (CB1) receptor and the subsequent discovery of the endogenous cannabinoid ligands (endocannabinoids), anandamide, 2-arachidonoyl glycerol (2-AG) and noladin ether, a intensive search for their function in health and disease has been launched. The endocannabinoids in the central nervous system bind Gi/o coupled CB1 receptors that modulate adenylyl cyclase, ion channels and extracellular signal-regulated kinases. The present review discusses the nature of endocannabinoid (anandamide and 2-AG) neurotransmission, the activity of cannabinoids and the possibility that some of these activities are mediated via a receptor, yet to be discovered, which is distinct from the brain specific CB1 receptor. Three physiological functions in which the endocannabinoids play a critical role are also discussed: embryonal implantation, feeding and appetite, and neuroprotection.

Down-regulation of cannabinoid-1 (CB-1) receptors in specific extrahypothalamic regions of rats with dietary obesity: a role for endogenous cannabinoids in driving appetite for palatable food?

Agonists at cannabinoid-1 (CB-1) receptors stimulate feeding and particularly enhance the reward aspects of eating. To investigate whether endogenous cannabinoids might influence appetite for palatable food, we compared CB-1 receptor density in the forebrain and hypothalamus, between rats fed standard chow (n=8) and others given palatable food (n=8) for 10 weeks to induce dietary obesity. CB-1 receptor density was significantly decreased by 30-50% (P<0.05) in the hippocampus, cortex, nucleus accumbens and entopeduncular nucleus of diet-fed rats. Furthermore, CB-1 receptor density in the hippocampus, nucleus accumbens and entopeduncular nucleus was significantly inversely correlated with intake of palatable food (r(2)=0.25-0.35; all P<0.05). By contrast, CB-1 receptor binding in the hypothalamus was low and not altered in diet-fed rats. CB-1 receptor down-regulation is consistent with increased activation of these receptors by endogenous cannabinoids. Acting in areas such as the nucleus accumbens and hippocampus, which are involved in the hedonic aspects of eating, cannabinoids may therefore drive appetite for palatable food and thus determine total energy intake and the severity of diet-induced obesity. However, cannabinoids in the hypothalamus do not appear to influence this aspect of eating behaviour.

The need to feed: homeostatic and hedonic control of eating

Feeding provides substrate for energy metabolism, which is vital to the survival of every living animal and therefore is subject to intense regulation by brain homeostatic and hedonic systems. Over the last decade, our understanding of the circuits and molecules involved in this process has changed dramatically, in large part due to the availability of animal models with genetic lesions. In this review, we examine the role played in homeostatic regulation of feeding by systemic mediators such as leptin and ghrelin, which act on brain systems utilizing neuropeptide Y, agouti-related peptide, melanocortins, orexins, and melanin concentrating hormone, among other mediators. We also examine the mechanisms for taste and reward systems that provide food with its intrinsically reinforcing properties and explore the links between the homeostatic and hedonic systems that ensure intake of adequate nutrition.

Inhibition of improgan antinociception by the cannabinoid (CB)(1) antagonist N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (SR141716A): lack of obligatory role for endocannabinoids acting at CB(1) receptors

Improgan, a nonopioid antinociceptive agent, activates descending, pain-relieving mechanisms in the brain stem, but the receptor for this compound has not been identified. Because cannabinoids also activate nonopioid analgesia by a brain stem action, experiments were performed to assess the significance of cannabinoid mechanisms in improgan antinociception. The cannabinoid CB(1) antagonist N-(piperidin-1-yl)-5-(4-chloro phenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (SR141716A) induced dose-dependent inhibition of improgan antinociception on the tail-flick test after i.c.v. administration in rats. The same treatments yielded comparable inhibition of cannabinoid [R-(+)-(2,3-dihydro-5-methyl-3-[(4-mor pholinyl)methyl]pyrol[1,2,3-de]-1,4-benzoxazin-6-yl)(1-naphthalenyl)methanone monomethanesulfonate, WIN 55,212-2] analgesia. Inhibition of improgan and WIN 55,212-2 antinociception by SR141716A was also observed in Swiss-Webster mice. Radioligand binding studies showed no appreciable affinity of improgan on rat brain, mouse brain, and human recombinant CB(1) receptors, ruling out a direct action at these sites. To test the hypothesis that CB(1) receptors indirectly participate in improgan signaling, the effects of improgan were assessed in mice with a null mutation of the CB(1) gene with and without SR141716A pretreatment. Surprisingly, improgan induced complete antinociception in both CB(1) (-/-) and wild-type control [CB(1) (+/+)] mice. Furthermore, SR141716A inhibited improgan antinociception in CB(1) (+/+) mice, but not in CB(1) (-/-) mice. Taken together, the results show that SR141716A reduces improgan antinociception, but neither cannabinoids nor CB(1) receptors seem to play an obligatory role in improgan signaling. Present and previous studies suggest that Delta(9)-tetrahydrocannabinol may act at both CB(1) and other receptors to relieve pain, but no evidence was found indicating that improgan uses either of these mechanisms. SR141716A will facilitate the study of improgan-like analgesics.

Cannabinoid receptor-G protein interactions: G(alphai1)-bound structures of IC3 and a mutant with altered G protein specificity

The structure of the C-terminal region of the third cytoplasmic loop (IC3) of the cannabinoid receptor one (CB1) bound to G(alphai1) has been determined using transferred nuclear Overhauser effects (NOEs). The wild-type IC3 sequence is helical when associated with G(alphai1). In contrast, a peptide containing the amino-acid inversion, Ala(341)-Leu(342) adopts a single turn. These findings correlate with the attenuated G(i) association of CB1 with the Ala(341)-Leu(342) mutation previously observed in vivo and the diminished stimulation of G(alphai1) GTPase activity by the corresponding peptide demonstrated in vitro here. These results, the first to report the structure of a GPCR domain while associated with G protein, imply the C-terminus of CB1 IC3, a region with high-sequence conservation among G-protein coupled receptors, must be helical for efficient coupling and activation of the G(i) protein.

DNA microarray analysis of cannabinoid signaling in mouse brain in vivo

To identify novel genes involved in cannabinoid receptor-mediated signaling, we used cDNA microarrays to detect changes in mRNA expression in the forebrains of mice 12 h after they were given a single intraperitoneal dose of the naturally-occurring Cannabis sativa alkaloid Delta(9)-tetrahydrocannabinol (Delta(9)-THC) or the synthetic cannabinoid receptor agonist (R)-(+)-2,3-dihydro-5-methyl-3-[(morpholinyl)methyl] pyrrolo[1,2,3-de]-1,4-benzoxazin-yl-1-naphtalenylmethanone mesylate [R(+)-WIN 55,212-2]. Of approximately 11,000 genes from a mouse brain cDNA library that were probed, 65 showed altered (increased or decreased at least 2-fold) expression after exposure to Delta(9)-THC, 41 after exposure to R(+)-WIN 55,212-2, and 20 genes after exposure to both drugs. Genes affected similarly by Delta(9)-THC and R(+)-WIN 55,212-2 were considered likely to reflect cannabinoid receptor activation, and expression of the protein products of two such genes not previously implicated in cannabinoid signaling-melanocyte-specific gene-related gene 1 (MRG1) and hexokinase 4 (glucokinase, GK)-was measured by Western blotting and immunohistochemistry. Western blots showed approximately 2-fold increases in the levels of both proteins in mouse forebrain. Immunohistochemistry revealed preferential localization of MRG1 to cerebral blood vessels and of GK to hypothalamic neurons. These findings suggest that MRG1 and GK are cannabinoid-regulated genes and that they may be involved in the vascular and hypothalamic effects of cannabinoids, respectively.

Oxygenated metabolites of anandamide and 2-arachidonoylglycerol: conformational analysis and interaction with cannabinoid receptors, membrane transporter, and fatty acid amide hydrolase

This study was aimed at finding structural requirements for the interaction of the acyl chain of endocannabinoids with cannabinoid receptors, membrane transporter protein, and fatty acid amide hydrolase (FAAH). To this end, the flexibility of the acyl chain was restricted by introduction of an 1-hydroxy-2Z,4E-pentadiene system in anandamide (N-arachidonoylethanolamine, AEA) and 2-arachidonoylglycerol (2-AG) at various positions using different lipoxygenases. This brought about selectivity and attenuated the binding potency of AEA and 2-AG. Although the displacement constants were modest, 15(S)-hydroxy-eicosa-5Z,8Z,11Z,13E-tetraenoyl-N-(2-hydroxyethyl)amine was found to bind selectively to the CB(1) receptor, whereas its 1-arachidonoyl-sn-glycerol analogue and 13(S)-hydroxy-octadeca-9Z,11E-dienoyl-N-(2-hydroxyethyl)amine could selectively bind to the CB(2) receptor. 11(S)-Hydroxy-eicosa-5Z,8Z,12E,14Z-tetraenoyl-N-(2-hydroxyethyl)amine did not bind to either receptor, whereas 12(S)-hydroxy-eicosa-5Z,8Z,10E,14Z-tetraenoyl-N-(2-hydroxyethyl)amine did bind to both CB receptors with an affinity similar to that of AEA. All oxygenated anandamide derivatives were good inhibitors of FAAH (low micromolar K(i)) but were ineffective on the AEA transporter. 2-AG rapidly isomerizes into 1(3)-arachidonoyl-sn-glycerol. Both 1- and 3-arachidonoyl-sn-glycerol did not bind to either CB receptor and did not interfere with AEA transport. Thus, after it is isomerized, 2-AG is inactivated, thereby decreasing effective concentrations of 2-AG. Analysis of (1)H NMR spectra revealed that chloroform did not induce notably different conformations in the acyl chain of 15(S)-hydroxy-eicosa-5Z,8Z,11Z,13E-tetraenoic acid as compared with water. Molecular dynamics (MD) simulations of AEA and its analogues in the presence of explicit water molecules revealed that a tightly folded conformation of the acyl chain is not the only requirement for CB(1) binding. Structural details of the C(2)-C(15) loop, such as an sp(2) carbon at position 11, are necessary for receptor binding. The MD simulations may suggest that the average orientations of the pentyl tail of AEA and 12(S)-hydroxy-eicosa-5Z,8Z,10E,14Z-tetraenoyl-N-(2-hydroxyethyl)amine are different from that of the low-affinity, inactive ligands.

Tetrahydrocannabinol and endocannabinoids in feeding and appetite

The physiological control of appetite and satiety, in which numerous neurotransmitters and neuropeptides play a role, is extremely complex. Here we describe the involvement of endocannabinoids in these processes. These endogenous neuromodulators enhance appetite in animals. The same effect is observed in animals and in humans with the psychotropic plant cannabinoid Delta(9)-tetrahydrocannabinol, which is an approved appetite-enhancing drug. The CB(1) cannabinoid receptor antagonist SR141716A blocks the effects on feeding produced by the endocannabinoids. If administered to mice pups, this antagonist blocks suckling. In obese humans, it causes weight reduction. Very little is known about the physiological and biochemical mechanisms involved in the effects of Delta(9)-tetrahydrocannabinol and the cannabinoids in feeding and appetite.

From cannabis to cannabinergics: new therapeutic opportunities

The molecular basis of cannabinoid activity is better understood since the discovery of the CB(1) receptor in the mammalian brain and the CB(2) receptor in peripheral tissues. Subsequently, an endogenous CB(1) receptor ligand, arachidonylethanolamide (anandamide), was isolated from porcine brain and shown to be metabolized by the enzyme arachidonylethanolamide amidohydrolase or fatty acid amide hydrolase. Recently, we have characterized a reuptake system for the transport of anandamide across the cell membrane, and have shown that selective inhibition of this transporter is associated with analgesia and peripheral vasodilation. The four cannabinoid system proteins, including the CB(1) and CB(2) receptors, fatty acid amide hydrolase, and the anandamide transporter, are excellent targets for the development of novel medications for various conditions, including pain, immunosuppression, peripheral vascular disease, appetite enhancement or suppression, and motor disorders. During the last decade, numerous selective ligands for each of these proteins were designed and synthesized. Many of these agents serve as important molecular probes, providing structural information about their binding sites, as well as pharmacological tools imparting information about the roles of their targets in physiological and disease states. All of the above compounds that modulate the functions of the endocannabinoid system can be collectively described under the term cannabinergics, regardless of chemical classification or type of resultant pharmacological action.

Is type 2 diabetes mellitus a disorder of the brain?

I propose that type 2 diabetes mellitus is due to damage to neurons in the ventromedial hypothalamus or to a defect in the action or properties of insulin or insulin receptors in the brain. These neuronal abnormalities are probably secondary to a marginal deficiency of long-chain polyunsaturated fatty acids during the critical periods of brain growth and development. Hence, supplementation of adequate amounts of long-chain polyunsaturated fatty acids during the third trimester of pregnancy to 2 y postterm can prevent or postpone the development of diabetes mellitus.

15-Lipoxygenase metabolism of 2-arachidonylglycerol. Generation of a peroxisome proliferator-activated receptor alpha agonist

The recent demonstrations that cyclooxygenase-2 and leukocyte-type 12-lipoxygenase (LOX) efficiently oxygenate 2-arachidonylglycerol (2-AG) prompted an investigation into related oxygenases capable of metabolizing this endogenous cannabinoid receptor ligand. We evaluated the ability of six LOXs to catalyze the hydroperoxidation of 2-AG. Soybean 15-LOX, rabbit reticulocyte 15-LOX, human 15-LOX-1, and human 15-LOX-2 oxygenate 2-AG, providing 15(S)-hydroperoxyeicosatetraenoic acid glyceryl ester. In contrast, potato and human 5-LOXs do not efficiently metabolize this endocannabinoid. Among a series of structurally related arachidonyl esters, arachidonylglycerols serve as the preferred substrates for 15-LOXs. Steady-state kinetic analysis demonstrates that both 15-LOX-1 and 15-LOX-2 oxygenate 2-AG comparably or preferably to arachidonic acid. Furthermore, 2-AG treatment of COS-7 cells transiently transfected with human 15-LOX expression vectors or normal human epidermal keratinocytes results in the production and extracellular release of 15-hydroxyeicosatetraenoic acid glyceryl ester (15-HETE-G), establishing that lipoxygenase metabolism of 2-AG occurs in an eukaryotic cellular environment. Investigations into the potential biological actions of 15-HETE-G indicate that this lipid, in contrast to its free-acid counterpart, acts as a peroxisome proliferator-activated receptor alpha agonist. The results demonstrate that 15-LOXs are capable of acting on 2-AG to provide 15-HETE-G and elucidate a potential role for endocannabinoid oxygenation in the generation of peroxisome proliferator-activated receptor alpha agonists.

Multiple neural systems controlling food intake and body weight

Discovery of the leptin receptor and its downstream peptidergic pathways has reconfirmed the crucial role of the hypothalamus in the regulation of food intake and energy balance. Strategically located in the midst of the mammalian neuraxis, the hypothalamus receives at least three distinct types of relevant information via direct or indirect neural connections as well as hormone receptors and substrate sensors bestowed on hypothalamic neurons. First, the medial and to a lesser extent the lateral hypothalamus receive a rich mix of information pertaining to the internal state of relative energy repletion/depletion. Second, specific hypothalamic nuclei receive information about the behavioral state, such as diurnal clock, physical activity-level, reproductive cycle, developmental stage, as well as imminent (e.g. fight and flight) and chronic (e.g. infection) stressors, that can potentially impact on short-term availability of fuels and long-term energy balance. Third, the hypothalamus, particularly its lateral aspects, receives information from areas in the forebrain involved in the acquisition, storage, and retrieval of sensory representations of the external food space and internal food experience, as well as from the executive forebrain involved in behavior selection and initiation. In addition, rich intrahypothalamic connections facilitate further distribution of incoming information to various hypothalamic nuclei. On the other hand, the hypothalamus has widespread neural projections to the same cortical areas it receives inputs, and many hypothalamic neurons are one synapse away from most endocrine systems and from both sympathetic and parasympathetic effector organs involved in the flux, storage, mobilization, and utilization of fuels. It is argued that processing within cortico-limbic areas and communication with hypothalamic areas are particularly important in human food intake control that is more and more guided by cognitive rather than metabolic aspects in the obesigenic environment of affluent societies. A distributed neural network for the control of food intake and energy balance consisting of a central processor and several parallel processing loops is hypothesized. Detailed neurochemical, anatomical, and functional analysis of reciprocal connections of the numerous peptidergic neuron populations in the hypothalamus with extrahypothalamic brain areas will be necessary to better understand what hypothalamus, forebrain, and brainstem tell each other and who is in charge under specific conditions of internal and external nutrient availability.

Update on anorexia and cachexia

Declining physical, emotional, and social function as a result of anorexia and cachexia are considerable contributors to discomfort for cancer patients and their families, and they impair the patient's ability to express optimal physical and psychosocial potential as long as possible. This decline no longer has to be accepted as an indispensable sequel to advanced cancer, just as pain is no longer considered to be unavoidable. A routine screening for anorexia and cachexia and associated symptoms is necessary, as is a careful, comprehensive assessment, because the condition is not always obvious. Decisions about anorexia and cachexia treatment are guided by prioritizing the different, concurrent physical, psychosocial, and existential problems and by considering the natural course of the cancer and the effects of antineoplastic therapies. Reversible causes for anorexia and cachexia need to be identified and treated, if appropriate. Nutritional interventions are often indicated; patients with a predominant starvation component and without inflammation may profit the most. New pharmacologic therapies for primary anorexia and cachexia syndrome are expected to enter clinical practice soon; however, until then, treatment with corticosteroids, progestins, or prokinetics may be indicated for some patients. To understand a multicausal syndrome, multimodal and interdisciplinary therapy is required. Specialist palliative care services can be helpful to provide, hand-in-hand with the disease specialists [172], assessment and management of psychophysical symptoms and sociospiritual needs of patients during the course of the illness and at the end of life [173]. Research efforts aim to better characterize subgroups of patients suffering from secondary causes of anorexia and cachexia and to elucidate the mechanisms involved in the primary anorexia and cachexia syndrome. Increasingly individualized treatments are expected with combination treatments that involve different mechanisms including nutrition.

Identification of the endogenous cannabinoid system through integrative pharmacological approaches

Scientific progress in the biological sciences increasingly relies on an integration of behavioral, pharmacological, cellular, and molecular approaches, particularly in translating basic research observations into therapeutic potential. The strength of in vivo model systems lies in the direct assessment of physiological function. However, they only allow indirect evidence for mechanism of action. Frequently, in vitro models provide just the opposite. A combination of both in vitro and in vivo approaches are often essential for establishing the underlying mechanisms of a specific pharmacological effect. In recent times, an endogenous cannabinoid system has been characterized due to the combined efforts of chemists, pharmacologists, molecular and cellular biologists, and biochemists. This endogenous cannabinoid system is providing a basis for systematically addressing the pharmacological controversies surrounding marijuana. The description of this endogenous cannabinoid system and the strategies for establishing the physiological function of this system are the subjects of this article.

Endocannabinoid levels in rat limbic forebrain and hypothalamus in relation to fasting, feeding and satiation: stimulation of eating by 2-arachidonoyl glycerol

Endocannabinoids are implicated in appetite and body weight regulation. In rodents, anandamide stimulates eating by actions at central CB1 receptors, and hypothalamic endocannabinoids may be under the negative control of leptin. However, changes to brain endocannabinoid levels in direct relation to feeding or changing nutritional status have not been investigated. We measured anandamide and 2-arachidonoyl glycerol (2-AG) levels in feeding-associated brain regions of rats, during fasting, feeding of a palatable food, or after satiation. Endocannabinoid levels were compared to those in rats fed ad libitum, at a point in their daily cycle when motivation to eat was absent. Fasting increased levels of anandamide and 2-AG in the limbic forebrain and, to a lesser extent, of 2-AG in the hypothalamus. By contrast, hypothalamic 2-AG declined as animals ate. No changes were detected in satiated rats. Endocannabinoid levels in the cerebellum, a control region not directly involved in the control of food intake, were unaffected by any manipulation. As 2-AG was most sensitive to variation during feeding, and to leptin regulation in a previous study, we examined the behavioural effects of 2-AG when injected into the nucleus accumbens shell, a limbic forebrain area strongly linked to eating motivation. 2-AG potently, and dose-dependently, stimulated feeding. This effect was attenuated by the CB1 receptor antagonist SR141716. These findings provide the first direct evidence of altered brain levels of endocannabinoids, and of 2-AG in particular, during fasting and feeding. The nature of these effects supports a role for endocannabinoids in the control of appetitive motivation.

Evaluation of CB1 receptor knockout mice in the Morris water maze

The endocannabinoid system has been proposed to modulate a variety of physiological processes, including those that underlie cognition. The present study tested whether this system is tonically active in learning and memory by comparing CB(1) receptor knockout mice (CB(1)(-/-)) to wild-type mice (CB(1)(+/+)) in several Morris water maze tasks. Also, the effects of three cannabinoid agonists, Delta(9)-tetrahydrocannabinol (Delta(9)-THC), R-(+)-[2,3-dihydro-5-methyl-3[morpholinyl)methyl]-pyrrolo[1,2,3-de]-1, 4-benzoxazinyl]-(1-naphthalenyl)methanone mesylate (WIN 55,212-2), and methanandamide, were evaluated in a working memory procedure. Both genotypes exhibited identical acquisition rates in a fixed platform procedure; however, the CB(1)(-/-) mice demonstrated significant deficits in a reversal task in which the location of the hidden platform was moved to the opposite side of the tank. This phenotype difference was most likely due to an increased perseverance of the CB(1)(-/-) mice in that they continued to return to the original platform location, despite being repeatedly shown the new platform location. In addition, Delta(9)-THC (ED(50) = 1.3 mg/kg), WIN 55,212-2 (ED(50) = 0.35 mg/kg), and methanandamide (ED(50) = 3.2 mg/kg) disrupted the performance of CB(1)(+/+) mice in the working memory task at doses that did not elicit motivational or sensorimotor impairment as assessed in a cued version of the task. Furthermore, doses of each drug that were maximally disruptive in CB(1)(+/+) mice were ineffective in either N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide HCl (SR 141716A)-treated CB(1)(+/+) or CB(1)(-/-) mice. These results provide strong evidence that cannabinoids disrupt working memory through a CB(1) receptor mechanism of action, and suggest that the endocannabinoid system may have a role in facilitating extinction and/or forgetting processes.

Behavioral neurobiology of alcohol addiction: recent advances and challenges

Addictive behavior associated with alcoholism is characterized by compulsive preoccupation with obtaining alcohol, loss of control over consumption, and development of tolerance and dependence, as well as impaired social and occupational functioning. Like other addictive disorders, alcoholism is characterized by chronic vulnerability to relapse after cessation of drinking. To understand the factors that compel some individuals to drink excessively, alcohol research has focused on the identification of brain mechanisms that support the reinforcing actions of alcohol and the progression of changes in neural function induced by chronic ethanol consumption that lead to the development of dependence. More recently, increasing attention has been directed toward the understanding of neurobiological and environmental factors in susceptibility to relapse.

Obesity therapy: altering the energy intake-and-expenditure balance sheet

Obesity is associated with numerous health complications, which range from non-fatal debilitating conditions such as osteoarthritis, to life-threatening chronic diseases such as coronary heart disease, diabetes and certain cancers. The psychological consequences of obesity can range from lowered self-esteem to clinical depression. Despite the high prevalence of obesity and the many advances in our understanding of how it develops, current therapies have persistently failed to achieve long-term success. This review focuses on how fat mass can be reduced by altering the balance between energy intake and expenditure.

Cannabinoid receptors and their ligands

There are at least two types of cannabinoid receptors, CB(1) and CB(2), both coupled to G proteins. CB(1) receptors exist primarily on central and peripheral neurons, one of their functions being to modulate neurotransmitter release. CB(2) receptors are present mainly on immune cells. Their roles are proving more difficult to establish but seem to include the modulation of cytokine release. Endogenous agonists for cannabinoid receptors (endocannabinoids) have also been discovered, the most important being arachidonoyl ethanolamide (anandamide), 2-arachidonoyl glycerol and 2-arachidonyl glyceryl ether. Other endocannabinoids and cannabinoid receptor types may also exist. Although anandamide can act through CB(1) and CB(2) receptors, it is also a vanilloid receptor agonist and some of its metabolites may possess yet other important modes of action. The discovery of the system of cannabinoid receptors and endocannabinoids that constitutes the "endocannabinoid system" has prompted the development of CB(1)- and CB(2)-selective agonists and antagonists/inverse agonists. CB(1)/CB(2) agonists are already used clinically, as anti-emetics or to stimulate appetite. Potential therapeutic uses of cannabinoid receptor agonists include the management of multiple sclerosis/spinal cord injury, pain, inflammatory disorders, glaucoma, bronchial asthma, vasodilation that accompanies advanced cirrhosis, and cancer. Following their release onto cannabinoid receptors, endocannabinoids are removed from the extracellular space by membrane transport and then degraded by intracellular enzymic hydrolysis. Inhibitors of both these processes have been developed. Such inhibitors have therapeutic potential as animal data suggest that released endocannabinoids mediate reductions both in inflammatory pain and in the spasticity and tremor of multiple sclerosis. So too have CB(1) receptor antagonists, for example for the suppression of appetite and the management of cognitive dysfunction or schizophrenia.

Endocannabinoids in cognition and dependence

Cannabis use is associated with a wide range of pharmacological effects, some of which have potential therapeutic benefit while others result in negative outcomes. Acute cannabinoid intoxication has been well documented to produce deficits in cognitive functioning with concomitant changes in glutamatergic, GABAergic, and cholinergic neurochemical systems in the hippocampus, each of which has been implicated in memory. Additionally, cannabis-dependent individuals abstaining from this drug can undergo a constellation of mild withdrawal effects. The use of the CB(1) cannabinoid receptor antagonist SR141716A and transgenic mice lacking the CB(1) receptor are critical tools for investigating the role of the endocannabinoid system in cognition, drug dependence, and other physiological processes. Converging evidence in which performance in a variety of memory tasks is enhanced following either SR141716A treatment or in CB(1) receptor knockout mice indicates that this system may play an important role in modulating cognition. There are also indications that this system may function to modulate opioid dependence. The purpose of this review is to describe recent advances that have furthered our understanding of the roles that the endocannabinoid system play on both cognition and drug dependence.

Discovery of endocannabinoids and some random thoughts on their possible roles in neuroprotection and aggression

A short history of the discovery of the main plant cannabinoid, Delta(9)-tetrahydrocannabinol and of the endogenous cannabinoids anandamide, 2-arachidonoyl glycerol and 2-arachidonyl glyceryl ether (noladin ether) is presented. The role of the cannabinoids in neuroprotection, with emphasis on the endocannabinoids, is described. The unexpected production of aggression by Cannabis and cannabinoids under stressful conditions, published mainly in the past, is summarized.

The role of endocannabinoids in the hypothalamic regulation of visceral function

The hypothalamus plays an important role in the regulation of several visceral processes, including food intake, thermoregulation and control of anterior pituitary secretion. Endogenous cannabinoids and CB(1) cannabinoid receptors have been found in the hypothalamus. In the present review, we would like to clarify the role of the endocannabinoid system in the regulation of the above-mentioned visceral functions. There is historical support for the role of marihuana (i.e. exogenous cannabinoids) in the regulation of appetite. Endocannabinoids also stimulate food intake. Furthermore, the specific CB(1) receptor antagonist SR141716 reduces food intake. Leptin treatment decreases endocannabinoid levels in normal rats and ob/ob mice. These findings provide evidence for the role of the hypothalamic endocannabinoid system in food intake and appetite regulation. Cannabinoids can change body temperature in a dose-dependent manner. High doses cause hypothermia while low doses cause hyperthermia. Cannabinoid administration decreases heat production. It seems that the effects of can- nabinoids on thermoregulation is exerted by altering some neurochemical mediator effects at both the presynaptic and postsynaptic level.THC and endocannabinoids have mainly inhibitory effects on the regulation of reproduction. Administration of anandamide (AEA) decreases serum luteinizing hormone (LH) and prolactin (PRL) levels. AEA causes a prolongation of pregnancy in rats and temporarily inhibits the postnatal development of the hypothalamo-pituitary axis in offspring. The action of AEA on the reproductory parameters occurs at both the hypothalamic and pituitary level. CB(1) receptors have also been found in the anterior pituitary. Further, LH levels in CB(1) receptor-inactivated mice were decreased compared with wild-type mice. Taken together, all these observations suggest that the endocannabinoid system is playing an important part in the regulation of the mentioned visceral functions and it provides the bases for further applications of cannabinoid receptor agonists and/or antagonists in visceral diseases regulated by the hypothalamus.

Biosynthesis and degradation of anandamide and 2-arachidonoylglycerol and their possible physiological significance

N -arachidonoylethanolamine (anandamide) was the first endogenous cannabinoid receptor ligand to be discovered. Dual synthetic pathways for anandamide have been proposed. One is the formation from free arachidonic acid and ethanolamine, and the other is the formation from N -arachidonoyl phosphatidylethanolamine (PE) through the action of a phosphodiesterase. These pathways, however, do not appear to be able to generate a large amount of anandamide, at least under physiological conditions. The generation of anandamide from free arachidonic acid and ethanolamine is catalyzed by a degrading enzyme anandamide amidohydrolase/fatty acid amide hydrolase operating in reverse and requires large amounts of substrates. As for the second pathway, arachidonic acids esterified at the 1-position of glycerophospholipids, which are mostly esterified at the 2-position, are utilized for the formation of N -arachidonoyl PE, a stored precursor form of anandamide. In fact, the actual levels of anandamide in various tissues are generally low except in a few cases. 2-Arachidonoylglycerol (2-AG) was the second endogenous cannabinoid receptor ligand to be discovered. 2-AG is a degradation product of arachidonic acid-containing glycerophospholipids such as inositol phospholipids. Several investigators have demonstrated that 2-AG is produced in a variety of tissues and cells upon stimulation. 2-AG acts as a full agonist at the cannabinoid receptors (CB1 and CB2). Evidence is gradually accumulating and indicates that 2-AG is the most efficacious endogenous natural ligand for the cannabinoid receptors. In this review, we summarize the tissue levels, biosynthesis, degradation and possible physiological significance of two endogenous cannabimimetic molecules, anandamide and 2-AG.

Molecular biology of cannabinoid receptors

During the last decade, research on the molecular biology and genetics of cannabinoid receptors has led to a remarkable progress in understanding of the endogenous cannabinoid system, which functions in a plethora of physiological processes in the animal. At present, two types of cannabinoid receptors have been cloned from many vertebrates, and three endogenous ligands (the endocannabinoids arachidonoyl ethanolamide, 2-arachidonoyl glycerol and 2-arachidonoyl-glycerol ether) have been characterized. Cannabinoid receptor type 1 (CB(1)) is expressed predominantly in the central and peripheral nervous system, while cannabinoid receptor type 2 (CB(2)) is present almost exclusively in immune cells. Cannabinoid receptors have not yet been cloned from invertebrates, but binding proteins for endocannabinoids, endocannabinoids and metabolic enzyme activity have been described in a variety of invertebrates except for molting invertebrates such as Caenorhabditis elegans and Drosophila. In the central nervous system of mammals, there is strong evidence emerging that the CB(1) and its ligands comprise a neuromodulatory system functionally interacting with other neurotransmitter systems. Furthermore, the presynaptic localization of CB(1) together with the results obtained from electrophysiological experiments strengthen the notion that in cerebellum and hippocampus and possibly in other regions of the central nervous system, endocannabinoids may act as retrograde messengers to suppress neurotransmitter release at the presynaptic site. Many recent studies using genetically modified mouse lines which lack CB(1) and/or CB(2) finally could show the importance of cannabinoid receptors in animal physiology and will contribute to unravel the full complexity of the cannabinoid system.

Endocannabinoids in the central nervous system--an overview

Many aspects of the physiology and pharmacology of anandamide (arachidonoyl ethanol amide), the first endogenous cannabinoid ligand ("endocannabinoid") isolated from pig brain, have been studied since its discovery in 1992. Ethanol amides from other fatty acids have also been identified as endocannabinoids with similar in vivo and in vitro pharmacological properties. 2-Arachidonoyl glycerol and noladin ether (2-arachidonyl glyceryl ether), isolated in 1995 and 2001, respectively, so far, display pharmacological properties in the central nervous system, similar to those of anandamide. The endocannabinoids are widely distributed in brain, they are synthesized and released upon neuronal stimulation, undergo reuptake and are hydrolyzed intracellularly by fatty acid amide hydrolase (FAAH). For therapeutic purposes, inhibitors of FAAH may provide more specific cannabinoid activities than direct agonists, and several such molecules have already been developed. Pharmacological effects of the endocannabinoids are very similar, yet not identical, to those of the plant-derived and synthetic cannabinoid receptor ligands. In addition to pharmacokinetic explanations, direct or indirect interactions with other receptors have been considered to explain some of these differences, including activities at serotonin and GABA receptors. Binding affinities for other receptors such as the vanilloid receptor, have to be taken into account in order to fully understand endocannabinoid physiology. Moreover, possible interactions with receptors for the lysophosphatidic acids deserve attention in future studies. Endocannabinoids have been implicated in a variety of physiological functions. The areas of central activities include pain reduction, motor regulation, learning/memory, and reward. Finally, the role of the endocannabinoid system in appetite stimulation in the adult organism, and perhaps more importantly, its critical involvement in milk ingestion and survival of the newborn, may not only further our understanding of the physiology of food intake and growth, but may also find therapeutic applications in wasting disease and infant's "failure to thrive".

Reversal of delta 9-THC hyperphagia by SR141716 and naloxone but not dexfenfluramine

Presatiated adult male Lister hooded rats received oral administration of the exogenous cannabinoid Delta-9-tetrahydrocannabinol (Delta(9)-THC; 1.0 mg/kg) in combination with subcutaneous injection of either the cannabinoid CB1 antagonist N-piperidino-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methylpyrazole-3-carboxamide (SR141716; 0, 0.05, 0.1, 0.5 or 1.0 mg/kg), the CB2 antagonist N-[(1S)-endo-1,3,3-trimethyl bicyclo[2.2.1]heptan-2-yl]-5-(4-chloro-3-methylphenyl)-1-(4-methylbenzyl)-pyrazole-3-carboxamide (SR144528; 0, 0.05, 0.1, 0.5 or 1.0 mg/kg), the general opioid antagonist naloxone (0.1, 0.5, 1.0 or 5.0 mg/kg) or the 5-HT agonist dexfenfluramine (0.05, 0.1, 0.5, 1.0 or 5.0 mg/kg). Food (chow) intake was measured over 2 h from the onset of the dark period. Delta(9)-THC induced significant hyperphagia, which was attenuated by subanorectic doses of SR141716 and naloxone. Neither SR144528 nor dexfenfluramine affected Delta(9)-THC-induced feeding. These data confirm mediation of Delta(9)-THC hyperphagia by central-type CB1 receptors, and support a functional relationship between cannabinoid and opioid systems in relation to appetite regulation. Stimulation of CB1 receptors may promote feeding by actions on food reward rather than by inhibition of serotonergic satiety mechanisms.

Design, synthesis and biological evaluation of novel arachidonic acid derivatives as highly potent and selective endocannabinoid transporter inhibitors

In the present work, we have designed and synthesized a series of arachidonic acid derivatives of general structure I which have been characterized as highly potent and selective inhibitors of anandamide transporter (IC(50) = 24-0.8 microM, K(i) > 1000-5000 nM for CB(1) and CB(2) cannabinoid receptors and vanilloid VR(1) receptor). Among them, N-(3-furylmethyl)eicosa-5,8,11,14-tetraenamide deserves special attention as being the most potent endocannabinoid transporter inhibitor (IC(50) = 0.8 microM) described to date.

Delta9 -tetrahydrocannabinol increases brain temperature and inverts circadian rhythms

Delta9-tetrahydrocannabinol (THC) has been shown to protect against focal and global ischemia. Hypothermia is thought to be one mechanism for this protection. These observations are important since brain hyperthermia is known to increase ischemic damage while hypothermia is protective. To establish the effect of THC on brain and body core temperature, brain and body temperature probes were inserted for chronic temperature monitoring (n = 20). THC treated groups were administered THC at either low (0.1 mg/kg) or high (10 mg/kg) dose for 1 week. Brain temperature was recorded during this period and for 1 week following the discontinuation of THC. Chronic administration of THC at either dose increased brain temperature (p < 0.0001) but did not significantly change body core temperature (p = 0.4767) in the freely moving rat.

Exogenous anandamide protects rat brain against acute neuronal injury in vivo

The endocannabinoid anandamide [N-arachidonoylethanolamine (AEA)] is thought to function as an endogenous protective factor of the brain against acute neuronal damage. However, this has never been tested in an in vivo model of acute brain injury. Here, we show in a longitudinal pharmacological magnetic resonance imaging study that exogenously administered AEA dose-dependently reduced neuronal damage in neonatal rats injected intracerebrally with the Na(+)/K(+)-ATPase inhibitor ouabain. At 15 min after injury, AEA (10 mg/kg) administered 30 min before ouabain injection reduced the volume of cytotoxic edema by 43 +/- 15% in a manner insensitive to the cannabinoid CB(1) receptor antagonist SR141716A. At 7 d after ouabain treatment, 64 +/- 24% less neuronal damage was observed in AEA-treated (10 mg/kg) rats compared with control animals. Coadministration of SR141716A prevented the neuroprotective actions of AEA at this end point. In addition, (1) no increase in AEA and 2-arachidonoylglycerol levels was detected at 2, 8, or 24 hr after ouabain injection; (2) application of SR141716A alone did not increase the lesion volume at days 0 and 7; and (3) the AEA-uptake inhibitor, VDM11, did not affect the lesion volume. These data indicate that there was no endogenous endocannabinoid tone controlling the acute neuronal damage induced by ouabain. Although our data seem to question a possible role of the endogenous cannabinoid system in establishing a brain defense system in our model, AEA may be used as a structural template to develop neuroprotective agents.

Control by the endogenous cannabinoid system of ras oncogene-dependent tumor growth

We investigated the effect of 2-methyl-arachidonyl-2'-fluoro-ethylamide (Met-F-AEA), a stable analog of the endocannabinoid anandamide, on a rat thyroid epithelial cell line (FRTL-5) transformed by the K-ras oncogene, and on epithelial tumors derived from these cells. Met-F-AEA effect in vivo was evaluated in a nude mouse xenograft model, where K-ras-transformed (KiMol) cells were implanted subcutaneously. Met-F-AEA (0.5 mg/kg/dose) induced a drastic reduction in tumor volume. This effect was inhibited by the CB1 receptor antagonist SR141716A (0.7 mg/kg/dose) and was accompanied by a strong reduction of K-ras activity. Accordingly, KiMol cells and tumors express CB1 receptors. Met-F-AEA inhibited (IC50 ~5 mM) the proliferation in vitro and the transition to the S phase of KiMol cells and it reduced K-ras activity; these effects were antagonized by SR141716A. Met-F-AEA cytostatic action was significantly smaller in nontransformed FRTL-5 cells than in KiMol cells. Met-F-AEA treatment exerted opposite effects on the expression of CB1 receptors in KiMol and FRTL-5 cells, with a strong up-regulation in the former case and a suppression in nontransformed cells. The data suggest that: 1) Met-F-AEA inhibits ras oncogene-dependent tumor growth in vivo through CB1 cannabinoid receptors; and 2) responsiveness of FRTL-5 cells to endocannabinoids depends on whether or not they are transformed by K-ras.

Anandamide administration into the ventromedial hypothalamus stimulates appetite in rats

This investigation reports the possible role of the endocannabinoid anandamide in modulating appetitive behaviour. Given that cannabinoids have been used clinically to stimulate appetite in HIV and cancer chemotherapy patients, there has been a renewed interest in the involvement of cannabinoids in appetite modulation. This is the first report on the administration of anandamide into the ventromedial hypothalamus. Pre-satiated rats received an intrahypothalamic injection of anandamide (50 ng x 0.5 microl(-1)) followed by measurement of food intake at 3 h post injection. Administration of anandamide induced significant hyperphagia. Pretreatment with the selective CB1 cannabinoid antagonist SR 141716 (30 microg x 0.5 microl(-1)), 30 min prior to anandamide injection resulted in an attenuation of the anandamide-induced hyperphagia (P<0.001). This study demonstrates that intrahypothalamic anandamide initiates appetite by stimulation of CB1 receptors, thus providing evidence on the involvement of hypothalamic endocannabinoids in appetite initiation.

Novel physiologic functions of endocannabinoids as revealed through the use of mutant mice

The presence in the mammalian brain of specific receptors for marijuana triggered a search for endogenous ligands, several of which have been recently identified. There has been growing interest in the possible physiological functions of endocannabinoids, and mutant mice that lack cannabinoid receptors have become an important tool in the search for such functions. To date, studies using CB1 knockout mice have supported the possible role of endocannabinoids in retrograde synaptic inhibition in the hippocampus, in long-term potentiation and memory, in the development of opiate dependence, and in the control of appetite and food intake. They also suggested the existence of as yet unidentified cannabinoid receptors in the cardiovascular and central nervous systems. The use of CB2 receptor knockout mice suggested a role for this receptor in macrophage-mediated helper T cell activation. Further studies will undoubtedly reveal many additional roles for this novel signaling system.

Endocannabinoids Part II: pathological CNS conditions involving the endocannabinoid system and their possible treatment with endocannabinoid-based drugs

Changes in the levels of either the cannabinoid CB(1) receptors or of their endogenous ligands, anandamide and 2-arachidonoylglycerol, appear to be casual or consequential in many neurological disorders. Several examples of how such diseases may be treated by substances capable of selectively manipulating endocannabinoid levels and action are presented, using animal models of neuropathological conditions, such as motor disorders, multiple sclerosis, neuronal damage, chronic and inflammatory pain, anorexia, cachexia and motivational disturbances. These examples indicate that new therapeutic agents, lacking the undesirable psychotropic side effects of Cannabis, may be developed from current studies on the endocannabinoid system.