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Three years later a second endocannabinoid, 2-arachidonoylglycerol 2-AG , was discovered independently by Mechoulam et al. Since then, a number of related endogenous lipids with endocannabinoid-like activity have been reported Fig.
The biochemical aspects of endocannabinoids have been recently reviewed by Bisogno et al. Anandamide is a partial or full agonist of CB 1 receptors, depending on the tissue and biological response measured. Although it also binds CB 2 receptors, it has very low efficacy and may act as an antagonist Gonsiorek et al. The in vivo biosynthesis of anandamide Fig. It is not yet known, however, whether NAPE-PLD is obligatory for the biosynthesis of anandamide, which could make it an attractive target of drug therapy when reduction of tissue anandamide would be of benefit.
Indeed, there may be parallel pathways for the generation of anandamide from NAPE. An alternative parallel pathway has been identified in our laboratory in RAW This latter pathway rather than PLD is the target of regulation by bacterial endotoxin, which increases anandamide synthesis in macrophages Varga et al. The existence of this pathway may also account for the recent finding that anandamide tissue levels are unchanged in NAPE-PLD knockout compared with wild-type mice Leung et al.
Schematic representation of the endocannabinoid system in pre- and postsynaptic neurons. The presynaptic terminal is located in the top, whereas the postsynaptic neuron is located in the bottom. DAG, an intracellular second messenger that activates protein kinase C, can be generated from phosphoinositides by a phosphoinositide-specific PLC or from phosphatidic acid by phosphatidic acid phosphohydrolase Bisogno et al.
In the adult brain they are localized in the postsynaptic plasma membrane, in line with their putative role in generating 2-AG involved in retrograde transmission. Basal levels of 2-AG in the brain are approximately 2 orders of magnitude higher than the levels of anandamide. Despite this, stimulus-induced release resulting in detectable extracellular levels could be demonstrated only for anandamide and not for 2-AG in an in vivo microdialysis study Giuffrida et al.
This finding illustrates that, despite growing interest in endocannabinoids and their roles as retrograde neurotransmitters Wilson and Nicoll, ; Chevaleyre et al. Like prostanoids, endocannabinoids are not stored but generated on demand in response to a depolarization-induced rise in intracellular calcium or activation of various metabotropic receptors Varma et al.
A putative membrane endocannabinoid transporter involved in the cellular uptake of endocannabinoids see below may also be involved in their release. This is suggested by the ability of a transport inhibitor to prevent the release of intracellularly applied anandamide Maccarrone et al.
Anandamide present in the extracellular space is accumulated by neurons and other cells by facilitated diffusion. This process is driven by its transmembrane concentration gradient, is saturable and temperature-dependent, and does not require ATP or sodium ions. Most importantly for the topic of the present review, anandamide uptake is selectively inhibited by a variety of structural analogs, which suggests the existence of a saturable cellular component involved in anandamide transport Beltramo et al.
However, a specific anandamide transporter protein has yet to be cloned, and it has been proposed that intracellular degradation of anandamide by fatty acid amide hydrolase FAAH is sufficient to account for anandamide uptake in long incubation periods Glaser et al. This notion is also supported by the emergence of a number of synthetic transport inhibitors, the potencies of which to inhibit anandamide uptake does not correlate with their affinities for CB 1 , CB 2 , or TRPV 1 receptors or their potencies to inhibit FAAH Fig.
However, in view of the important role of FAAH in generating the transmembrane concentration gradient for anandamide, the possibility that a noncatalytic region of FAAH or a FAAH-associated protein may act as anandamide transporter cannot be excluded. Interestingly, the elucidation of the crystal structure of FAAH revealed several channel-like regions in the enzyme, granting it simultaneous access to both the cytosolic and membrane domains Bracey et al. Arguments for and against the existence of a bidirectional anandamide transporter have been recently reviewed Hillard and Jarrahian, ; Fowler et al.
The structure and pharmacological specificity of inhibitors of FAAH and of endocannabinoid membrane transport. In some in vivo studies, treatment with transport inhibitors unmasked cannabinoid-like tonic effects on pain sensitivity, anxiety-like behaviors, locomotor activity, and muscle spasticity, which is an indication of the potential therapeutic usefulness of such compounds Moore et al.
Similar and more pronounced effects have been reported in response to treatment with FAAH inhibitors, as discussed below. In contrast to the unsettled status of anandamide transport and a putative transporter protein, the unique role of FAAH in the in vivo degradation of anandamide has been extensively documented reviewed in McKinney and Cravatt, Initial evidence for a membrane-associated enzyme in the liver that hydrolyzes N - N -acyl ethanolamides Schmid et al.
The unique role of FAAH in terminating signaling by anandamide was indicated by the phenotype of FAAH knockout mice, which displayed 10 to 15 times elevated levels of anandamide across the brain, supersensitivity to the actions of exogenous anandamide, and the appearance of tonic signaling by endogenous anandamide, resulting in CB 1 receptor-mediated hypoalgesia Cravatt et al.
These mice did not display the hypoalgesia observed in mice with global deficiency in FAAH, but had a similar anti-inflammatory phenotype, indicating that the latter was mediated by elevated fatty acid amides in peripheral tissues Cravatt et al. Interestingly, another amidohydrolase catalyzing the same reaction as FAAH but at acidic pH was recently identified and cloned Tsuboi et al.
This lysosomal enzyme is structurally unrelated to FAAH and is widely distributed in tissues, with highest levels in the lung, and has been recently shown to contribute to the physiological degradation of anandamide in macrophages but not in the brain Sun et al. However, the behavioral consequences of DSI and its modulation remain unclear: Therefore, at this point it is difficult to predict the potential therapeutic usefulness of selective MGL inhibitors.
It has been known since antiquity that use of cannabis in its various forms increases appetite, particularly for palatable foods, and can also result in significant weight gain Donovan, ; Berry and Mechoulam, Following the identification of THC as the main psychoactive principle in marijuana, the appetite-promoting effect of smoked marijuana could be attributed to THC even before the identification of specific cannabinoid receptors Hollister, ; Greenberg et al.
Variability in the observed changes in THC-induced food intake may also relate to the feeding state of the animal, the orexigenic effect being optimal in presatiated animals with low basal levels of food intake Williams et al.
After the discovery of specific cannabinoid receptors and the introduction of selective antagonists, the increase in food intake caused by THC could be linked to CB 1 receptors, as it was blocked by the selective CB 1 antagonist SR, but not by the CB 2 antagonist SR Williams and Kirkham, The discovery of endocannabinoids has raised the question of their potential involvement in the physiological control of appetite and energy metabolism.
This subject has been the topic of a number of recent reviews Di Marzo and Matias, ; Kirkham, ; Sharkey and Pittman, ; Pagotto et al. The first indication of a role for endocannabinoids in appetite control was the documented ability of low doses of anandamide to increase food intake, when administered either systemically Williams and Kirkham, ; Hao et al. Similar increases in food intake can be elicited by 2-AG administered systemically or into the nucleus accumbens shell region Kirkham et al.
Sites for the orexigenic actions of endocannabinoids in both the hypothalamus and the limbic forebrain suggest their involvement in both the homeostatic and hedonic control of eating Harrold and Williams, ; Vickers and Kennett, Interestingly, endocannabinoid activation of hypothalamic centers, such as the paraventricular nucleus, may also occur indirectly via CB 1 receptors on peripheral afferent nerve terminals Gomez et al. Studies with antagonists provide more direct support for a regulatory function of endocannabinoids on feeding.
Definitive evidence for the involvement of endocannabinoids in the control of food intake has been provided through the use of CB 1 receptor-deficient mice.
In a study from our laboratory, food-deprived CB 1 knockout mice were found to eat less than their wild-type littermates, and their food intake was unaffected by SR treatment, whereas in wild-type mice SR reduced food intake to the levels seen in the knockout mice Di Marzo et al. Similar findings have been subsequently reported by others Wiley et al. This indicates that part of the hunger-induced increase in food intake is mediated by endocannabinoids acting at CB 1 receptors.
This latter finding suggests that the absence of leptin results in increased endocannabinoid activity. Indeed, hypothalamic levels of endocannabinoids were elevated in leptin-deficient mice and rats and reduced after leptin treatment, suggesting that endocannabinoids are part of the leptin-regulated neural circuitry involved in appetite regulation Di Marzo et al.
Endogenous leptin may similarly suppress endocannabinoid levels, as indicated by our recent unpublished findings using mice with obesity induced by a high-fat diet, which have elevated plasma leptin levels proportional to their increased fat mass.
Anandamide levels were significantly lower in the obese mice compared with their lean controls in the hypothalamus, limbic forebrain, and amygdala, with no difference in the cerebellum. Furthermore, there was a significant inverse correlation between plasma leptin levels and anandamide levels in the above three brain regions involved in appetite control but not in the cerebellum. A possible hypothalamic site for an interaction between leptin and endocannabinoids is the lateral hypothalamus, where CB 1 receptors are present in orexin-and melanin-concentrating hormone MCH -containing neurons Cota et al.
These neurons project to dopaminergic neurons in the ventral tegmental area Fadel and Deutch, , where they modulate the mesolimbic dopaminergic pathway involved in food reward. Thus, they could also represent a site of integration of hypothalamic and extrahypothalamic structures involved in the orexigenic effect of endocannabinoids. The resulting increase in the activity of MCH neurons may contribute to the in vivo appetitive effect of endocannabinoids. Furthermore, this DSI could be blocked by leptin through inhibition of voltage-gated calcium channels in the MCH neurons, whereas it was increased 6-fold in leptin-deficient mice Jo et al.
At this site, glucocorticoids have been shown to induce endocannabinoid synthesis and endocannabinoid-induced suppression of synaptic excitation via a cAMP-dependent mechanism, and leptin was found to block these effects by a phosphodiesterase 3B-mediated decrease in intracellular cAMP Malcher-Lopes et al. These effects may underlie the orexigenic action of glucocorticoids. Another recent study indicates the importance of lateral hypothalamic orexin neurons in reward-seeking behavior in general Harris et al.
Additionally, cannabinoids can increase the intake of palatable foods by acting at sites in the brainstem Miller et al. From a behavioral point of view, cannabinoids are involved in both the appetitive and consummatory aspects of feeding behavior Chaperon et al.
Such multiple sites of action are also indicated by findings that in THC-naive rats, rimonabant suppressed food-maintained operant responses and metabolic activity in the limbic forebrain, measured by 2-deoxyglucose uptake, whereas in rats made tolerant to THC, an additional metabolic inhibition was detected in the hypothalamus Freedland et al. Exposure of rats to a palatable diet containing sucrose and condensed milk resulted in down-regulation of CB 1 receptors in limbic structures involved in the hedonic aspects of feeding, but not in the hypothalamus Harrold et al.
In the hypothalamus, the very low density of CB 1 receptors is offset by their increased coupling Breivogel et al. Indeed, CB 1 -deficient mice fail to self-administer morphine Ledent et al.
The observation that SR inhibits the orexigenic effect of morphine microinjected into the hypothalamic paraventricular nucleus but not the nucleus accumbens shell suggests additional interactions between the two systems, unrelated to the hedonic aspects of feeding Verty et al. However, anandamide was found to increase and AM to decrease depolarization-induced NPY release in rat hypothalamic explants, suggesting that NPY may contribute to the orexigenic effects of cannabinoids Gamber et al.
A possible role of orexins in the appetitive effects of endocannabinoids is suggested by the finding that coexpression of the CB 1 and orexin 1 receptors results in a marked potentiation of orexin A-induced signaling Hilairet et al. An important site of action of the orexigenic peptide ghrelin is the hypothalamic paraventricular nucleus, where its hyperphagic effect can be blocked by SR, suggesting that ghrelin may act via the release of endocannabinoids Tucci et al.
Endocannabinoids, in turn, may be involved in ghrelin release, at least in the periphery, as suggested by an SRinduced decrease in plasma ghrelin levels in rats Cani et al. Furthermore, mice deficient in FAAH have reduced levels of CART immunoreactivity in various hypothalamic and extrahypothalamic regions involved in appetite control, which is returned to normal levels by chronic SR treatment Osei-Hyiaman et al.
These findings suggest that inhibition of CART release by CB 1 activation may be involved in the orexigenic effect of anandamide. It is generally accepted that energy intake and utilization are regulated in a coordinated fashion, and factors involved in the central regulation of appetite may also affect peripheral energy metabolism Seeley and Woods, The first indirect indication that cannabinoids may affect energy homeostasis through a mechanism other than food intake came from a study of marijuana smokers tested in a hospital inpatient setting Greenberg et al.
In this study, the marijuana-induced increase in caloric intake leveled off after a few days, whereas weight gain continued throughout the rest of the day observation period, suggesting independent effects on appetite and peripheral energy metabolism. Tolerance to the anorectic effect of SR developed within 5 days, whereas the reduction in body weight was maintained throughout the treatment period Colombo et al. Later, similar observations were reported in mice with diet-induced obesity, in which food intake was reduced transiently whereas the reduction in body weight was maintained when the animals were chronically treated with SR Ravinet Trillou et al.
These results suggested that factors other than appetite must be involved in the weight-reducing effect of CB 1 antagonists. Peripheral targets of endocannabinoids include adipocytes, which express CB 1 receptors Bensaid et al. Stimulation of CB 1 receptors on adipocytes can affect lipid metabolism through regulating the level of adiponectin production Bensaid et al. The work by Cota et al. By careful analysis of body composition, they were able to establish the lean phenotype of CB 1 -deficient mice that had escaped earlier attention.
Furthermore, the use of a pair-feeding paradigm revealed that hypophagia accounts for the lean phenotype only in young and not in adult animals, which clearly indicated the involvement of peripheral metabolic target s in the latter.
The additional documentation of functional CB 1 receptors in primary cultured adipocytes and their role in regulating lipogenesis provided one of the likely peripheral targets for the anabolic effects of endocannabinoids.
Although earlier studies failed to detect CB 1 receptors in the liver, more recently they have been identified in the mouse liver using a combination of methods including reverse transcription-polymerase chain reaction, in situ hybridization, immunohistochemistry, and Western blotting. In the same study, treatment of mice with the cannabinoid agonist HU increased de novo lipogenesis and the expression of the transcription factor sterol regulatory element binding protein 1c SREBP1c as well as of its targets, ACC1 and fatty acid synthase Osei-Hyiaman et al.
The role of CB 1 receptors in these effects was indicated by the ability of SR to block them and by their absence in CB 1 knockout mice Osei-Hyiaman et al. The hepatic lipogenic pathway may be also directly activated through a cannabinoid-induced decrease in AMPK phosphorylation and activity in the liver Kola et al.
CB 1 receptors have been also detected in rat hepatocytes Michalopoulos et al. Fatty acid metabolism in hypothalamic neurons acts as a sensor of nutrient availability Obici et al.
Although fatty acid synthesis was not measured directly in the hypothalamus, these findings suggest that the increase in food intake after fasting may involve a CB 1 -mediated modulation of the fatty acid synthetic pathway. Modulation of AMPK activity by cannabinoids was documented not only in liver and adipose tissue but also in hypothalamus Kola et al.
This suggests that the high-fat diet-induced decrease in energy expenditure is mediated by endocannabinoid activation of CB 1 receptors. Expression of the thermogenic uncoupling protein-1 was also down-regulated by CB 1 activation, whereas the expression of the insulin-mimetic adipokine visfatin was increased Perwitz et al. Conversely, rimonabant increases adiponectin secretion by adipocytes Bensaid et al. Glucose uptake, subsequently measured in the isolated soleus muscle of these animals, was significantly increased in the SRpretreated group.
A similar effect in humans may account for the increased glucose tolerance observed in obese patients treated with rimonabant Van Gaal et al. These observations could suggest the presence of CB 1 receptors in skeletal muscle, which was recently documented Pagotto et al. Alternatively, increased glucose tolerance may be secondary to an effect of SR on CB 1 receptors in the liver.
It has been proposed that increased lipid synthesis in the liver may produce insulin resistance in other tissues such as muscle McGarry, , and CB 1 receptor activation has been shown to contribute to the development of hepatic steatosis in diet-induced obesity Osei-Hyiaman et al. Rimonabant-induced decreases in body weight and adipose tissue mass in obese mice was accompanied by a near-complete reversal of obesity-induced changes in the expression of a wide range of genes.
These included genes involved in adipocyte differentiation, lipolysis, generation of futile cycles, and glycolysis. These broad-based targets may underlie the ability of rimonabant to correct symptoms of the metabolic syndrome, as discussed below. They also raise the intriguing possibility that if a CB 1 antagonist that does not cross the blood-brain barrier were available, it could be effective in the treatment of the metabolic syndrome without the risk of adverse CNS side effects Horvath, Genetic manipulation of the expression of endogenous proteins has been instrumental in uncovering their regulatory role in normal and pathological phenotypes.
When CB 1 knockout mice were first introduced, no change in body mass or feeding pattern had been noted Ledent et al. However, in a subsequent study, CB 1 knockout mice were found to have a life-long, small, but significant, weight deficit compared with their wild-type littermates, which could be attributed to a selective deficit in adipose tissue mass Cota et al. The possibility that an increase in the activity of the endocannabinoid system may contribute to at least some forms of obesity was suggested by three sets of findings.
First, CB 1 antagonists were significantly more efficacious in reducing caloric intake and body weight in rodents with diet-induced or genetic obesity than in their respective lean controls Di Marzo et al. As a third line of evidence, recent findings indicate that endocannabinoids and CB 1 receptors are up-regulated in the liver and adipose tissue in various forms of experimental as well as in human obesity. In wild-type mice on a high-fat diet for 3 weeks, the basal rate of de novo hepatic fatty acid synthesis was markedly increased, and the increase was partially reversed by SR treatment Osei-Hyiaman et al.
After 3 weeks of diet, the mice were not yet overweight but showed significant hepatic steatosis. Their hepatic content of anandamide was increased 3-fold, and the level of CB 1 receptor protein in liver plasma membranes was also markedly increased Osei-Hyiaman et al.
These findings indicate that intake of a high-fat diet activates the hepatic endocannabinoid system, which contributes to increased lipogenesis and the subsequent development of hepatic steatosis and, ultimately, the development of obesity.
CB 1 receptor knockout mice are resistant to these diet-induced changes, which indicates that endocannabinoids have a major role in mediating them Osei-Hyiaman et al. An up-regulation of CB 1 receptors has been also reported in adipose tissue of genetically obese compared with lean mice Bensaid et al.
In a study involving 40 women Engeli et al. Although these plasma levels were much too low to exert hormone-like activity, they probably originate from overflow from tissues and thus may reflect functionally relevant changes in endocannabinoid content at or near sites of action.
In the same study, FAAH expression was markedly reduced in the adipose tissue of obese subjects and correlated negatively with circulating endocannabinoid levels. These findings suggest that the endocannabinoid system is activated in human obesity Engeli et al.
A genetic missense polymorphism in the FAAH gene predicting a proline to threonine substitution at position , which was reported to result in reduced cellular expression and activity of the enzyme Chiang et al. The same polymorphism has been linked to overweight and obesity in both Caucasian and African-American subjects Sipe et al. Interestingly, the elevated hepatic levels of anandamide in mice receiving a high-fat diet could be attributed to a decrease in FAAH activity Osei-Hyiaman et al.
The opposing effects of elevated levels of both anandamide and oleylethanolamide after pharmacological blockade of FAAH may therefore result in no net change in appetite and energy metabolism. That increased endocannabinoid activity may also contribute to obesity and its metabolic consequences in humans was indicated by the highly promising results of recent clinical trials with rimonabant. As in the animal models of diet-induced obesity, rimonabant was effective both in reducing body weight and in reversing many of the associated metabolic abnormalities in obese subjects.
Blood pressure was not significantly affected. The parallel reduction in body weight and waist circumference suggested that the weight loss was predominantly due to loss of visceral fat, which is known to be a predisposing factor for the metabolic syndrome.
Rimonabant was well tolerated, with mild to moderate nausea, diarrhea, and mood disorders occurring slightly more in the treatment group than in the placebo group Van Gaal et al. Essentially similar findings were reported in another large-scale, phase III study RIO-North America involving randomized, obese or overweight subjects. At the end of the 1st year, rimonabant-treated subjects were re-randomized to receive rimonabant or placebo, whereas the placebo group continued onto receive the placebo.
During the 2nd year, rimonabant-treated patients retained the improvements achieved during the 1st year, whereas those who switched to placebo regained their original weight Pi-Sunyer et al. It is possible that a modest reduction in blood pressure by rimonabant occurs preferentially in males. The cumulative finding that blood pressure reduction, if present, is less than expected based on a similar level of weight reduction alone Appel et al.
As discussed in section D. Although the pressor effect is much smaller at lower doses of rimonabant comparable with the mg oral dose used in humans or in the absence of anesthesia S. Kunos, unpublished observations , careful monitoring of blood pressure, particularly in the early stages of rimonabant treatment, may be advisable.
It is noteworthy that part of the rimonabant-induced improvements in the hormonal and lipid abnormalities in the three clinical studies appeared to be independent of weight reduction and, based on the preclinical findings discussed above, are most likely mediated via peripheral sites of action. An interesting alternative mechanism is suggested by the results of a recent meta-analysis of the effects of low carbohydrate, nonenergy-restricted diets on weight loss and cardiovascular risk factors Nordmann et al.
Such diets were found to lead to significant weight loss for up to 1 year. Surprisingly, they were more favorable than low-fat diets in reducing plasma triglycerides and increasing HDL cholesterol levels, without a favorable effect on total or LDL cholesterol. The pattern of these metabolic changes is similar to that of those caused by 20 mg of rimonabant in the three clinical trials.
Rimonabant has been shown to preferentially suppress the preference for sweet compared with normal Simiand et al. It is very possible that obese subjects treated with rimonabant unwittingly altered their diet by reducing carbohydrate intake, which may have contributed to the observed effects on triglycerides and HDL cholesterol. Detailed analyses of the effects of rimonabant on dietary habits are warranted. Overall, the findings in these three large, multicenter clinical trials strongly support a pathogenic role of increased endocannabinoid activity in obesity and the associated metabolic abnormalities and highlight the unique therapeutic potential of CB 1 blockade.
Additional benefits may be gained by combination therapies. The efficacy of statins to preferentially lower LDL cholesterol may be effectively complemented by the ability of rimonabant to increase HDL cholesterol. In the case of insulin, the ability of rimonabant ability to increase insulin sensitivity could reduce the dose requirement for insulin in obese diabetic subjects and could also counteract the tendency of insulin treatment to cause weight gain.
Nevertheless, further large-scale studies are warranted in view of the high nonadherence rate observed in the three clinical trials to date, which may have resulted in overestimation of the benefits of treatment Simons-Morton et al. Although there is a growing body of evidence documenting the therapeutic effectiveness of synthetic THC or even smoked marijuana as appetite boosters in some of these conditions Regelson et al.
A few studies have reported the effectiveness of THC in stimulating appetite and weight gain in cancer patients, but these therapeutic effects have been more extensively documented in AIDS patients reviewed by Kirkham, ; Martin and Wiley, ; Hall et al.
Although concerns have been voiced about the potential immunosuppressive effect of cannabinoids in immunocompromised individuals Klein et al. Anorexia may also be associated with normal aging. A number of hormonal factors have been implicated in the loss of appetite in the elderly, including growth hormone, cholecystokinin, leptin, and various cytokines Morley, These findings suggest that, at least in this animal model, an age-dependent decrease in CB 1 receptor signaling in the limbic forebrain may be related to the parallel decline in appetite for both food and alcohol.
Anorexia nervosa is a psychiatric condition that occurs predominantly in younger women and is characterized by self-starvation, weight loss, and a disturbed body image.
Plasma anandamide levels have been reported to increase in patients with restricting anorexia nervosa, which may be secondary to a marked decrease in plasma leptin levels in such patients Monteleone et al.
Although the relationship between brain and plasma levels of anandamide is not clear, a parallel increase in anandamide in brain regions involved in reward may mediate the rewarding effect of self-starvation in anorexic patients Monteleone et al. A recent family-based study examined the possible association of a CB 1 receptor gene polymorphism consisting of differences in a trinucleotide repeat with anorexia nervosa.
Endocannabinoids have been also implicated in a unique form of food intake: In an elegant series of studies, Fride et al.
As predicted by this model, treatment of pups with SR inhibits suckling and leads to death due to failure to thrive, a condition analogous to a human condition known as nonorganic failure to thrive, in which an oral motor defect resulting in deficient suckling Reilly et al. The relatively high dose of SR to inhibit suckling and its residual effectiveness in CB 1 knockout mice suggested the additional involvement of a receptor distinct from CB 1 or CB 2 Fride et al.
One of the earliest uses of cannabis was to treat pain. Historical documents reveal the use of cannabis for surgical anesthesia in ancient China and to relieve pain of diverse origin in ancient Israel, Greece, Rome, and India reviewed in Mechoulam, ; Iversen, ; Mechoulam and Hanus, Numerous early studies have also demonstrated beneficial effects of cannabinoids in animal models of pain reviewed in Walker and Huang, ; Fox and Bevan, Recent animal studies indicate that anandamide and cannabinoid ligands are also very effective against chronic pain of both neuropathic Herzberg et al.
Moreover, endocannabinoids and synthetic cannabinoids exert synergistic antinociceptive effects when combined with commonly used nonsteroid anti-inflammatory drugs, which may have utility in the pharmacotherapy of pain Guindon and Beaulieu, ; Guindon et al. Interestingly, a recent study has implicated the endocannabinoid system in the analgesic activity of paracetamol acetaminophen , the most widely used painkiller Ottani et al.
Cannabinoids exert their antinociceptive effects by complex mechanisms involving effects on the central nervous system Martin et al. This is consistent with the anatomical location of CB 1 receptors in areas relevant to pain in the brain, spinal dorsal horn, dorsal root ganglia, and peripheral afferent neurons Herkenham et al. In addition to the role of CB 1 receptors, there is recent evidence implicating CB 2 receptors in the antihyperalgesic activity of cannabinoids in models of acute and chronic, neuropathic pain, especially of inflammatory origin Calignano et al.
Cannabinoid agonists may also release endogenous opioids, and a functional interplay between the endocannabinoid and opioid systems in modulating analgesic responses has been suggested by numerous studies Pugh et al. As discussed before, anandamide is also a ligand for TRPV 1 receptors, albeit with an affinity lower than its affinity for CB 1 receptors. The potential involvement of TRPV 1 in the analgesic effect of endogenous anandamide has been raised by the findings that the analgesic response to microinjection of a FAAH antagonist into the periaqueductal gray of rats could be inhibited by a similar local microinjection of 6 nmol of capsazepine Maione et al.
The analgesic response to exogenous cannabinoids suggested a role for the endocannabinoid system in regulating pain sensitivity, which has received experimental support reviewed in Walker et al. For example, Walker et al. The functional role of endogenous anandamide was further supported by the predominantly CB 1 -mediated analgesic response to FAAH or endocannabinoid transport inhibitors in animal models of acute and chronic pain Lichtman et al.
Similarly, FAAH knockout mice had elevated brain levels of anandamide and displayed analgesic behavior in acute inflammatory, but not in chronic neuropathic models of pain Lichtman et al.
Formation of anandamide and 2-AG is also increased in response to stress in the periaqueductal gray matter, in which inhibition of endocannabinoid degradation was found to enhance stress-induced analgesia in a CB 1 receptor-dependent manner Hohmann et al. In humans, the analgesic activity of THC and other cannabinoids is less clear-cut. There are numerous case reports on the beneficial effects of cannabis or synthetic derivatives of THC in pain associated with multiple sclerosis, cancer, neuropathies, and HIV infection Noyes et al.
The results of randomized studies conducted before on the analgesic effect of orally administered synthetic cannabinoids in patients with postoperative, post-traumatic, cancer, or spastic pain had been subjected to a meta-analysis. The authors concluded that cannabinoids were not more effective than codeine in controlling pain, and their use was associated with numerous undesirable, dose-limiting CNS side effects Campbell et al.
Recent clinical trials with THC or cannabis extracts containing a 1: In a randomized, double-blind, placebo-controlled crossover study of 48 patients suffering from central neuropathic pain due to brachial plexus avulsion, oromucosally administered THC or Sativex was ineffective in reducing the pain severity score recorded during the last 7 days of treatment Berman et al.
Similarly, oral THC dronabinol did not improve postoperative Buggy et al. However, numerous lessons have been learned from these initial human studies on optimal trial design, dose and route of administration of cannabinoids, and more recent larger-scale studies allow reason for more optimism, as outlined below.
THC or Sativex reduced neuropathic pain in patients with traumatic nerve injury or multiple sclerosis in randomized, double-blind, placebo-controlled, crossover trials Wade et al. Modest, but clinically relevant analgesic effects were reported in 21 multiple sclerosis patients treated with dronabinol, in a randomized, controlled clinical trial Svendsen et al. Effective pain relief by orally administered cannabis extract or THC was also reported in a randomized, controlled, multicenter trial involving multiple sclerosis patients Zajicek et al.
Moreover, in a recent study of 66 multiple sclerosis patients, Sativex was effective in reducing central neuropathic pain Rog et al. A preview of as-yet-unpublished human studies gave an account of a significant benefit of Sativex over placebo in peripheral neuropathic pain characterized by allodynia, in central pain associated with multiple sclerosis, and in opiate-resistant, intractable pain due to cancer Russo, A multicenter dose-escalation study of the analgesic and adverse effects of an oral cannabis extract Cannador in patients with postoperative pain demonstrated significant dose-related improvements in rescue analgesia requirements and significant trends across the escalating dose groups for decreasing pain intensity Holdcroft et al.
THC Marinol was found to suppress otherwise intractable cholestatic pruritus in a case report Neff et al. The therapeutic potential of cannabinoids in pain associated with trigeminal neuralgia and migraine has also been the subject of several recent reviews Liang et al. Preclinical studies Burstein et al. Numerous additional human studies are ongoing to determine the effectiveness of THC or cannabis-based extracts against various forms of pain reviewed in Ware and Beaulieu, reviewed in Ware and Beaulieu, Multiple lines of evidence support the important role of the cannabinoid signaling system in the modulation of immune function and inflammation reviewed in Klein et al.
First, cannabinoid receptors are present on immune cells, where their expression is modulated by microbial antigens or other stimuli that induce immune activation. Second, stimulation of immune cells by bacterial toxins such as lipopolysaccharide LPS increases the cellular levels of endocannabinoids and their degrading enzyme s. Third, cannabinoid agonists modulate immune function both in vitro and in vivo via cannabinoid receptor-dependent and -independent mechanisms.
The anti-inflammatory effects of cannabinoids are complex and may involve modulation of cytokine e. To the extent that pain and inflammation accompany many of the disorders discussed in the rest of this review, cannabinoids would be expected to provide significant benefit due to their analgesic and anti-inflammatory properties.
The emerging role of the endocannabinoid system in a variety of CNS disorders should not come as a surprise given the very high level of expression of CB 1 receptors in the brain. The particularly high density of CB 1 receptors in the cortex, cerebellum, hippocampus, and basal ganglia had drawn early attention to diseases affecting movement, mood and anxiety disorders, and conditions related to altered brain reward mechanisms, as well as processes of memory and learning.
The classic behavioral effects of marijuana also provided early clues about potential therapeutic targets, such as the control of pain or appetite. The role of the endocannabinoid system in the pathogenesis and treatment of specific CNS diseases is discussed below. The endocannabinoid system plays an important role in neuroprotection both in acute neuronal injury e. Although the underlying mechanisms are not fully understood, multiple cannabinoid receptor-dependent as well as receptor-independent processes have been implicated.
These include, but are not limited to 1 modulation of excitatory glutamatergic transmissions and synaptic plasticity via presynaptic CB 1 receptors Molina-Holgado et al. Excitotoxicity, the toxic effects of an overactivation of glutamate receptors, and the resulting oxidative stress may contribute to the pathological processes eventually leading to cellular dysfunction or death in both acute and chronic forms of neurodegeneration Coyle and Puttfarcken, ; McNamara, ; Lutz, Dexanabinol HU , a behaviorally inactive cannabinoid and noncompetitive antagonist of NMDA receptors, protects primary rat neuronal cultures against NMDA and glutamate exposure in vitro Eshhar et al.
THC protects primary cultured neurons against kainate-mediated toxicity in a CB 1 -dependent manner Abood et al. Palmitoylethanolamide also improves neuronal survival in a glutamate-induced cell death model Skaper et al. Intracerebral injection of NMDA in neonatal rats results in a fold increase of cortical anandamide concentrations Hansen et al. Both THC and anandamide exerted CB 1 -mediated neuroprotective effects in an ouabain-induced rat model of in vivo excitotoxicity van de Stelt et al.
Anandamide and synthetic agonists of CB 1 receptors also protected the newborn brain against AMPA-kainate receptor-mediated excitotoxic damage in mice Shouman et al.
Traumatic brain injury TBI is one of the leading causes of disability and mortality in young individuals Holm et al. TBI is characterized by cerebral edema, axonal and neuronal injury, increased permeability of the blood-brain barrier, and post-traumatic changes in cognitive and neurological functions Bayir et al. TBI can trigger glutamate-induced excitotoxicity, oxidative stress, release of inflammatory cytokines from brain-resident cells microglia, neurons, and astrocytes , programmed cell death, and cortical blood flow dysregulation reviewed in Wang and Feuerstein, ; Gentleman et al.
The protective effect of cannabinoids in traumatic brain injury was first indicated in studies with the nonpsychotropic cannabinoid dexanabinol HU Fig. These studies have demonstrated reduced brain damage and improved motor and cognitive function in HUtreated animals in a rat model of TBI. The favorable effects of a single injection of HU on learning and neurological deficits lasted up to 30 days and could be achieved within a therapeutic window of 6 h Shohami et al.
Beneficial effects of HU were also demonstrated in an axonal injury model Yoles et al. In mice with closed head injury, brain levels of 2-AG increased, and exogenous 2-AG administered 1 h after the head injury reduced infarct size and improved neurological outcome Panikashvili et al.
A multicenter, double-blind, randomized, placebo-controlled phase II trial conducted in 67 patients with severe closed head injury found dexanabinol to be safe and well tolerated. However, a double-blind, randomized, placebo-controlled phase III clinical trial of dexanabinol, conducted in 15 countries in 86 specialized centers and involving patients failed to demonstrate any favorable effects Maas et al.
Ischemic stroke is the most common form of stroke, mostly caused by a transient interruption of blood supply to the brain by thrombotic occlusion of blood vessels. It is an important cause of death and disability in industrialized countries, affecting up to 0.
One in six patients die in the 1st month after ischemic stroke, and half of the survivors are permanently disabled despite the best efforts to rehabilitate them and to prevent complications Klijn and Hankey, One of the first indications of the neuroprotective effect of cannabinoids came from the field of stroke research, using various in vitro and in vivo models of ischemic injury. Anandamide, 2-AG, and WIN 55, protected cultured cortical neurons against hypoxia and glucose deprivation Nagayama et al.
The effects of various cannabinoid ligands were also investigated in in vivo models of global cerebral ischemia induced by two-vessel occlusion with hypotension or by four-vessel occlusion, or in focal ischemia induced by occlusion of the middle cerebral artery MCAo , with or without reperfusion.
Importantly, this protective effect was observed even when the drug was administered 60 to min after the insult Vered et al. WIN 55,, at doses of 0. It also reduced infarct size after permanent focal cerebral ischemia induced by MCAo, when given 40 min before 30 min after the occlusion, and these effects were prevented by SR Nagayama et al.
WIN 55, also protected cultured cerebral cortical neurons from in vitro hypoxia and glucose deprivation, but in contrast to the receptor-mediated neuroprotection observed in vivo, this in vitro effect was not stereoselective and was insensitive to CB 1 and CB 2 receptor antagonists Nagayama et al.
BAY also decreased infarct size in rats with permanent MCAo even when given intravenously 4 h after the occlusion Mauler et al. The protective effect of HU was partially reversed by pretreatment with SR, indicating CB 1 receptor involvement. Surprisingly, all protection could be abolished by warming the animals to the body temperature of controls, indicating that CB 1 -mediated hypothermia contributed to the neuroprotection Leker et al.
Consistent with these findings, CB 1 knockout mice had increased mortality from permanent focal cerebral ischemia, increased infarct size, more severe neurological deficits after transient focal cerebral ischemia, and decreased cerebral blood flow in the ischemic penumbra during reperfusion, compared with wild type controls subjected to the same insult Parmentier-Batteur et al.
Further evidence for a role of CB 1 receptors is their increased expression on neurons in the arterial boundary zone of cortical infarction Jin et al. Finally, brain levels of endocannabinoids are increased during ischemic Schmid et al. Other studies do not support the neuroprotective role of CB 1 receptor activation.
Thus, it appears that both CB 1 agonists and antagonists can be neuroprotective in cerebral ischemia. The reason for the opposite effects of pharmacological blockade versus genetic knockout of CB 1 receptors is not clear and may be related to the CB 1 receptor-independent effects of antagonists Begg et al.
Clearly, evaluating the potential usefulness of cannabinoid ligands in the treatment of stroke warrants future studies. Multiple sclerosis MS is a complex, immune-mediated, inflammatory disease of the white matter of the brain, which compromises impulse conduction due to the loss of the myelin sheath of neurons and the secondary axonal loss Sospedra and Martin, MS affects 2 to 5 million people worldwide and commonly presents with an unpredictable, relapsing-remitting course and a range of clinical symptoms depending on where the demyelination and axonal loss have occurred Compston and Coles, Some patients become disabled within a short period of time, whereas others can live their entire lives with only negligible or no disability.
The symptoms of MS typically involve tremor, ataxia, visual loss, double vision, weakness or paralysis, difficulty in speaking, loss of bladder control and constipation, cognitive impairment, and painful muscle spasms.
Muscle spasticity often leads to reduced mobility, considerable distress from pain, and interference with daily living activities. Spasticity, neuropathic and nociceptive pain, and some of the above symptoms are also common in spinal cord injury SCI. Although there are numerous drugs available that target the immune system to slow down the progression of the disease, they are only moderately effective, and the treatment of MS remains mostly symptomatic and far from satisfactory Killestein and Polman, Cannabis had been used in ancient Greece, Rome, China, and India for relieving muscle cramps, spasm, and pain reviewed in Mechoulam, , Mechoulam et al.
THC treatment not only reduced CNS inflammation and improved neurological outcome but also improved survival compared with placebo. The nonpsychotropic dexanabinol also suppressed inflammatory responses in the brain and spinal cord of rats with EAE and improved their neurological symptoms Achiron et al. Although CB 1 receptor density is decreased in the striatum and cortex of EAE rats, this is compensated for by increased coupling to G protein-mediated signaling, ensuring the effectiveness of treatment with cannabinoid agonists Berrendero et al.
Furthermore, spasticity could be relieved not only by administration of exogenous anandamide, 2-AG, or PEA but also by selective inhibitors of endocannabinoid transport or hydrolysis, which suggests tonic control of muscle tone by the endocannabinoid system in EAE Baker et al. Additional evidence for this has emerged through the use of CB 1 -deficient mice, which tolerated inflammatory and excitotoxic insults poorly and developed substantial neurodegeneration after the induction of EAE Pryce et al.
Interestingly, CB 1 knockout mice had increased caspase 3 levels before the induction of EAE, suggesting a neuroprotective tone mediated by CB 1 receptors Jackson et al. These changes were paralleled by extensive remyelination Arevalo-Martin et al.
Consistent with the animal data, cannabinoids have shown promise in the treatment of MS in humans Table 1. There have been anecdotal reports of the effectiveness of marijuana smoking in relieving symptoms of MS and SPI Grinspoon and Bakalar, , , which were supported by the results of early open or single-blind observations with orally given THC or smoked marijuana, involving small numbers of patients Dunn and Davis, ; Petro, ; Petro and Ellenberger, ; Clifford, ; Meinck et al.
The most consistent finding was a subjective improvement in spasticity, although benefits for mobility, tremor, nystagmus, mood, and bladder control were also reported. In a double-blind crossover study of a single MS patient, nabilone treatment improved muscle spasms, nocturia, and general well-being Martyn, In contrast, Greenberg et al. These encouraging reports have triggered numerous larger, population-based clinical trials of cannabis-based medicines in MS, which have yielded mixed results.
Using a randomized, double-blind, placebo-controlled, crossover design, Killestein et al. Spasticity and disability, quantified using the objective Ashworth scale Ashworth, and the Expanded Disability Status Scale were not improved.
However, a significant improvement in the subjective rating of spasm frequency and trends toward improved mobility were noted, with no effect on tremor, sleep quality, or lower urinary tract symptoms.
It should be mentioned, however, that the dose of THC used was lower than that in subsequent studies with more positive outcome, and as was noted in an accompanying editorial Thompson and Baker, , the study was not powered to detect efficacy. There was no change in Ashworth score, tremor, irritability, depression, or tiredness after 15 weeks of treatment with Marinol or Cannador.
However, there were significant improvements in patient-reported spasticity, pain, and sleep quality. Unexpectedly, there was also a reduction in hospital admissions for relapse in the two active treatment groups. Adverse side effects were generally minor and similar to those with placebo. Remarkably, in the month follow-up of the original CAMS study of patients, muscle spasticity measured by the Ashworth scale was significantly improved in the THC-treated group.
The Rivermead Mobility Index was also improved, indicative of reduced disability. The effect of Cannador on tremor was also studied in a randomized, double-blind, placebo-controlled, crossover trial in 14 patients with MS. Consistent with an earlier report Zajicek et al. In another study of similar design, administration of oral capsules containing 2. Endocannabinoids are released upon demand from lipid precursors in a receptor-dependent manner and serve as retrograde signalling messengers in GABAergic and glutamatergic synapses, as well as modulators of postsynaptic transmission, interacting with other neurotransmitters, including dopamine.
Endocannabinoids are transported into cells by a specific uptake system and degraded by two well-characterized enzymes, the fatty acid amide hydrolase and the monoacylglycerol lipase. Recent pharmacological advances have led to the synthesis of cannabinoid receptor agonists and antagonists, anandamide uptake blockers and potent, selective inhibitors of endocannabinoid degradation.
These new tools have enabled the study of the physiological roles played by the endocannabinoids and have opened up new strategies in the treatment of pain, obesity, neurological diseases including multiple sclerosis, emotional disturbances such as anxiety and other psychiatric disorders including drug addiction. Recent advances have specifically linked the endogenous cannabinoid system to alcoholism, and cannabinoid receptor antagonism now emerges as a promising therapeutic alternative for alcohol dependence and relapse.
Received 1 September ; first review notified 17 September ; in revised form 22 September ; accepted 1 October The extensive research on the structure and activity of the natural constituents of Cannabis termed cannabinoids and the development of synthetic compounds with high potency and stereoselectivity have led to the identification of the main physiological functions that are modulated by this new class of drugs Howlett et al.
The discovery of the cannabinoid receptor and the availability of highly selective and potent cannabimimetics led to the rapid identification of a family of lipid transmitters that serve as natural ligands for the CB 1 receptor: The pharmacological properties of the endocannabinoids were found to be very similar to those of the synthetic cannabimimetics.
The subsequent description of a complex biochemical pathway for the synthesis, release Di Marzo et al. Since the discovery of anandamide, more than scientific reports have comprehensively explored the main aspects of the endocannabinoid system.
This system now appears as a relevant modulator of physiological functions not only in the central nervous system but also in the autonomic nervous system, the endocrine network, the immune system, the gastrointestinal tract, the reproductive system and in microcirculation Di Marzo et al. A selection of reviews and reports that explore in depth the main aspects of endocannabinoids and their receptors. Since the discovery of anandamide in , over publications have reported new data on the biological role of the endogenous cannabinoid system.
The present review gives a general perspective of the endogenous cannabinoid system, including the main pharmacological advances in the development of drugs capable of modulating their dynamics. The review focuses on the role of endocannabinoids as modulators of reward circuits and motivated behaviour that are relevant for drug addiction, including alcoholism. In light of the extensive research over the past 12 years, several specialized reviews wherein the reader will find a more profound analysis of the role played by the endocannabinoid system in selected physiological functions are shown in Table 1.
When discovered, the endocannabinoids were found to be derivatives of arachidonic acid, which resembled other lipid transmitters eicosanoids such as prostaglandins or leukotrienes. Additional studies revealed the existence of other structure-related lipid messengers including palmitylethanolamide or oleoylethanolamide, which are not active at cannabinoid receptors. These messengers will not be included in this review, although they serve important physiological functions in inflammation, pain control, feeding behaviour and lipid metabolism Calignano et al.
Endocannabinoids are derivatives of arachidonic acid conjugated with ethanolamine or glycerol. Figure 1 depicts the chemical structure of four endocannabinoids, anandamide, 2-arachidonoylglycerol 2-AG , the ester of arachidonic acid and ethanolamine; virodhamine which resembles anandamide Porter et al. All these endocannabinoids have been found in the brain, plasma and peripheral tissues, although the relevance of noladin has been questioned recently Oka et al.
In the brain, the concentration of anandamide is fold lower than that of 2-AG Sugiura et al. The monoglyceride 2-AG is a metabolic intermediate in lipid metabolism whereas anandamide is the product of the cleavage of a membrane phospholipid. However, after depolarization or receptor stimulation e.
Left, the structure of four arachidonic acid derivatives that have been identified as endogenous ligands for both the cannabinoid CB 1 and CB 2 receptors. Different pathways are involved in the synthesis and release of anandamide and 2-AG. Figure 2 shows the dynamics of formation and degradation of anandamide.
Overview of the biochemical pathways for synthesis, degradation and cellular actions of the endogenous cannabinoid anandamide. Anandamide action is terminated through a two-step process, which includes, first, its cellular uptake through a specific anandamide transporter AT and second, degradation by enzymatic cleavage to arachidonic acid AA and ethanolamide by the membrane-bound enzyme fatty acid amidohydrolase FAAH. Anandamide is formed by the cleavage of a phospholipid precursor, the N -arachidonoyl-phosphatidylethanolamine NAPE.
The precursor is synthesized by the enzyme N -acyltransferase NAT , which catalyses the transfer of arachidonic acid from phosphatidylcholine to the head group of phosphatidylethanolamine. This enzyme has no homology with the known PLD enzymes and is classified as a member of the zinc metallohydrolase family. Its presence is highest in the brain, kidneys and testis.
The synthesis and release of 2-AG is different from that of anandamide. Because 2-AG is a monoglyceride, its formation is closely associated with the metabolism of triacylglycerol, mainly by the receptor-dependent activation of phosphatidylinositol-specific phospholipase C PLC. The standard model proposes that activation of metabotropic receptors coupled to the PLC and diacylglycerol DG lipase pathway will systematically lead to increases in 2-AG production Stella et al.
Cloning of the enzyme 1,2-diacylglycerol lipase Bisogno et al. Once anandamide and 2-AG are formed, they target the CB 1 receptors in the same cell where they were formed, via diffusion within the plasmalemma, or they can be released to the extracellular fluid where they reach distant targets i. Endocannabinoid signalling is terminated by a two-step process that includes transport into cells and hydrolysis by two specific enzymatic systems. Both steps exert a tight control of endocannabinoid levels in tissues, rapidly eliminating these signalling molecules.
Endocannabinoid uptake is mediated by a transporter Beltramo et al. The transporter is an elusive molecule which works in a manner that is similar to other lipid carriers: The anandamide transporter is saturable, displays substrate specificity and can be blocked by specific drugs such as AM Fig.
A major issue of debate has been the potential coupling of endocannabinoid transport and degradation: However, a recent report seems to confirm that transport and degradation are independent processes Fegley et al.
The degradation of endocannabinoids is performed by two specific enzymatic systems: FAAH is a membrane enzyme that belongs to the serine—hydrolase family. FAAH is widely distributed throughout the body, with high concentrations in the brain and liver. FAAH can degrade many fatty acid amides, including acylethanolamides such as anandamide and the sleep factor oleamide. This enzyme is also a serine hydrolase and its distribution in the nerve terminals of specific brain neurons has been determined recently Gulyas et al.
Two major cannabinoid receptors have been cloned, both of which belong to the superfamily of G-protein-coupled receptors. The first receptor described was named the CB 1 receptor and it is mainly located in the terminals of nerve cells central and peripheral neurons and glial cells , the reproductive system i.
The CB 2 cannabinoid receptor was found initially in multiple lymphoid organs with the highest expression detected in B lymphocytes, moderate expression in monocytes and polymorphonuclear neutrophils and the lowest expression in T lymphocytes, although subsequent studies identified it in microglial cells as well Munro et al.
An interesting aspect of cannabinoid receptors is their expression during development of the brain, where they control cell differentiation Rueda et al.
Pharmacological studies revealed the existence of other endocannabinoid targets including the vanilloid receptor Zygmunt et al. The existence of these and other putative cannabinoid receptors, and their role in endocannabinoid physiology can be clarified only after their molecular characterization.
Cannabinoid receptors, especially the CB 1 receptor, display unique properties. The most relevant property is their preservation throughout evolution: The preservation of this ancient signalling system in vertebrates and several invertebrate phyla reflects the important functions played by the endocannabinoids in cell and system physiology.
A second remarkable characteristic of the CB 1 receptors is their high expression in the brain. The CB 1 receptor is the most abundant G-protein-coupled receptor, with densities 10—50 fold above those of classical transmitters such as dopamine or opioid receptors Howlett et al. Another important characteristic is the low efficiency of CB 1 receptor coupling to its transduction system: Both cannabinoid receptors are coupled to similar transduction systems.
Cannabinoid receptor activation was initially reported to inhibit cAMP formation through its coupling to Gi proteins Devane et al. These actions are relevant to the role of cannabinoids as modulators of neurotransmitter release Schlicker and Kathmann, and short-term synaptic plasticity Wilson and Nicoll, , as discussed below. Further research also described the coupling of CB 1 and CB 2 receptors to the mitogen-activated protein kinase cascade, to the phosphatidylinositol 3-kinase, to the focal adhesion kinase, to ceramide signalling and to nitric oxide production Derkinderen et al.
Finally, recent studies revealed that under certain conditions, the CB 1 receptors can stimulate formation of cAMP by coupling to the Gs protein Felder et al. Endocannabinoids exhibit different binding properties and intrinsic activity at CB 1 and CB 2 receptors. The intrinsic activity of anandamide at CB 1 receptors is 4—30 fold higher than at CB 2 receptors. As described above, the endogenous cannabinoid system is widely distributed throughout the body.
In the peripheral tissues the localization of the elements of the endogenous cannabinoid system reflects the distribution of the cell types where they are located e.
B lymphocytes in spleen and lymph nodes. However, in the nervous system the distribution is much more complex and structured, and clearly reflects the importance of this system in synaptic transmission. In some regions, such as the hippocampus, there is a complementary distribution of cannabinoid receptors, endocannabinoid transporters and degradation enzymes. However, in other areas of the brain, for instance the thalamus, there are discrepancies i.
From the early work of Herkenham et al. Binding studies and in situ hybridization analysis showed that the cannabinoid receptors are synthesized in somata and the protein transported to axon terminals Herkenham et al.
The phenotype of the CB 1 receptor-expressing neurons corresponds mainly to GABAergic neurons including cholecystokinin-containing neocortical, amygdalar and hippocampal neurons and dynorphin- and substance P-expressing medium spiny neurons of the outflow nuclei of basal ganglia Tsou et al.
Several glutamatergic and cholinergic telencephalic and cerebellar neurons also express the CB 1 receptors Piomelli, In the peripheral nervous system, the CB 1 receptors are located in sensory neurons of the dorsal root ganglia. Figure 3 shows how the CB 1 receptors are synthesized in medium spiny neurons of the caudate-putamen and the protein transported to the axon terminals in the globus pallidus and substantia nigra.
The dense presence of CB 1 binding sites in the cerebellum, hippocampus, striatum, globus pallidum and substantia nigra clearly reflects this biological characteristic of CB 1 receptors. Imaging cannabinoid CB 1 receptor in circuits of the rat brain reward system. Cannabinoid receptors are mainly located at presynaptic axon terminals.
However, the protein is mainly detected by immunohistochemistry panels C and D in the axon terminals innervating both outflow nuclei of the basal ganglia. In these areas, CB 1 receptors are not located in dopaminergic neurons Panel F: The segregation of CB 1 receptors and catecholaminergic transmission is also observed in the hippocampus-dentate gyrus Hpc-DG, panel G.
Fatty acid amide hydrolase is present in large principal neurons, such as the pyramidal cells of the cerebral cortex, the pyramidal cells of the hippocampus, the Purkinje cells of the cerebellar cortex and the mitral cells of the olfactory bulb. This complementary distribution suggests that FAAH closely controls the duration of cannabinoid effects, although there are sites where this association does not occur, such as the outflow nuclei of basal ganglia.
Monoglyceride lipase is located mainly in the hippocampus, cortex, cerebellum and anterior thalamus, with moderate expression in the extended amygdala, including the shell of the nucleus accumbens Dinh et al.
The spatial segregation of the two enzymes suggests that anandamide and 2-AG signalling may subserve functional roles that also involve spatial segregation, raising a controversy with respect to the nature and function of the retrograde endocannabinoid signal Gulyas et al.
The distribution of the anandamide transporter has been only partially characterized because the transporter has not been cloned. The distribution of transport activity is highest in areas expressing CB 1 receptors, such as the hippocampus, the amygdala, the striatum and the somatosensory, motor and limbic areas of the cortex.
Transport activity is also present in areas with low expression of the CB 1 receptor, such as the thalamus and the hypothalamus Beltramo et al. During the last twenty years, and especially after the discovery of the CB 1 receptor and anandamide, an intense research effort has yielded numerous series of drugs that interact with most of the main elements of the endogenous cannabinoid system.
Today we have drugs that bind to the CB 1 receptor as agonists or antagonists, drugs that block the endocannabinoid transport and drugs that inhibit the activity of FAAH.
Both in vitro and in vivo bioassays have been used to evaluate the activity of the new compounds. Prior to the availability of radioligand cannabinoid receptors, in vitro assays included the inhibition of forskolin-stimulated cAMP production and the inhibition of electrically evoked contractions of isolated smooth muscle preparations. Smooth muscle preparations most often used for the bioassay of cannabinoids are the mouse-isolated vas deferens and the myenteric plexus-longitudinal muscle preparation from the guinea pig small intestine.
These bioassays, which are particularly sensitive, rely on the ability of cannabinoid receptor agonists to act via the CB 1 receptors to inhibit electrically evoked contractions. In vivo bioassays include behavioural tests for analgesia and locomotion. What is the logic of a cannabinoid approach to pharmacotherapeutics? Cannabinoid receptor agonists may be designed to mimic the signalling processes mediated by anandamide and 2-AG, mainly in pathological situations where a boost in cannabinoid receptor stimulation might be needed.
Cannabinoid receptor antagonism might be the approach selected in conditions with enhanced endocannabinoid signalling. Transport inhibition and inhibition of degradation are more sophisticated approaches, both oriented towards magnifying the tonic actions of endocannabinoids.
A rational use of these therapeutic strategies requires the identification and evaluation of the functional status of endocannabinoid signalling in reference disorders. Thus, a deficit of anandamide signalling during conditions of stress might be counteracted by the blockade of anandamide degradation Kathuria et al. As a summary of cannabinoid pharmacology, Table 2 shows the reference compound for each molecular target, indicating Ki in the case of ligand—receptor interaction or IC50 in the case of enzymatic inhibitors.
Targeting the endogenous cannabinoid system: According to the International Union of Pharmacology reviewed in Howlett et al. New series of compounds have been recently described, including diarylether sulfonylesters Mauler et al. Classical cannabinoids are tricyclic dibenzopyran derivatives that are either compounds occurring naturally in the plant C. Non-classical cannabinoids are synthetic THC analogues that lack the dihydropyran ring.
The most representative form is the Pfizer compound CP , a potent and complete agonist at both the CB 1 and CB 2 receptors, which was used to characterize the CB 1 receptor for the first time Devane et al.
The Endocannabinoid System as an Emerging Target of Pharmacotherapy
As a consequence, the endocannabinoid system was discovered, research concerning the endocannabinoid-system is of importance in both. Herein, we review the pharmacology of the endocannabinoid system with a focus on its role in pain modulation. This article discusses clinical studies that have. The endocannabinoid system (ECS) plays a very important role in the For example, in a clinical trial conducted with schizophrenic patients.