Endocannabinoid System - Myth or Magic
Shrikant Srivastava, M.D. and David Nutt, Psychopharmacology Unit, University of Bristol, UK
Cannabis has been known to man for many centuries and in different cultures, has served different purposes - from being an offering to God Shiva in Hindu religion to drug of abuse in modern Western culture. In last fifty years, systematic investigations led to discovery of the active ingredient, delta-9 tetrahydrocannabinol (D-9THC) in 1960s, and in 1990s specific cannabis receptors were identified.
Obesity in Modern Society
Obesity is on the rise across the globe. The International Obesity Task Force estimates 1.1 billion to be overweight including 312 million who are obese.1 The prevalence in Europe is about 10-20% in men and 15-25% in women, and time-trends indicate increasing estimates. In US, the obesity rates increased from 15% in 1980 to 27% in 1999. Obesity as risk factor for burden of disease ranks among top 5 disorders in both developing and developed nations.
Pharmacotherapy to treat obesity is nothing new. Previously, d-amphetamine and other stimulants were used for this purpose, but their use was restricted for well-known reasons of dependence and abuse. The advent of Fluoxetine, a specific serotonin reuptake inhibitor, raised hopes, which died as it was discovered that its induced weight loss was not sustained. The development of Rimonabant, an antagonist at cannabis receptors, may usher an important era in psychopharmacology against obesity.
Pharmacology of Endocannabinoid (ECB) System
Two types of cannabis receptors – CB1 and CB2, have been cloned so far. CB1 are found in nerve cells (central and peripheral neurons and glial cells), the reproductive System (i.e. testis), and the microcirculation. The CB2 receptors are present in multiple lymphoid organs. The focus of this article is on CB1 receptors and their role in modulating appetite and body weight.
With our current understanding, cannabis receptors are most highly expressed G-protein coupled receptor in human brain, and there are more CB1 receptors in brain than the total of all dopamine, serotonin and noradrenaline receptors combined.2 Importantly there is marked homology of CB1 receptors through the evolutionary tree.3
Anandamide and 2-Arachidonoglycerol (2-AG) are the main endogenous ligands in brain for CB1 receptors. Synthesised from arachidonic acid [a major component of the fatty acids in clee membranes], these are released in calcium-dependant fashion and their action is terminated by uptake in the cells by a specific transporter in an energy-dependent fashion. Once inside the cell, the ECBs are degraded by specific enzyme Systems - fatty acid amino hydrolase FAAH) and monoacylglyceride lipase (MAGL). However, these endogenous ligands are not stored in vesicles, but immediately released following synthesis. 4
Anandamide behaves as a partial agonist at both CB1 and CB2 receptors, but has higher affinity and intrinsic activity for the CB1 receptor. However, 2-AG is a complete agonist but with less affinity than anandamide at both types of receptors5
Cannabis receptors inhibit the formation of c-AMP. In addition, or they also act on ion channels - to inhibit the inflow of Ca ++ ions in the cells and enhance inwardly rectifying K+ currents. The net effect is hyperpolarisation resulting in Inhibition of neuronal activity. Endocannabinoids acting on pre-synaptic CB1 receptors thus can have stimulating effect, for if they are located on GABA-ergic terminals they can turn off these inhibitory cells so disinhibiting the brain.
Overall, endocannabinoids act as local messengers that contribute significantly to the elimination of Information flow through specific synapses. CB1 receptor Stimulation has a major impact on second messengers involved in synaptic remodeling, and neuronal differentiation and survival. Thus the ECB System works as a major homeostatic mechanism fine tuning the Information processing in the brain and has counter-regulatory mechanisms aimed at preserving the structure and function of major brain circuits. They are involved in maintaining homoeostasis like motivated behavior (feeding, reproduction, relaxation, sleep) and emotions and cognitions. Additionally, they are also involved in modulation of implicit learning of motor routines through basal ganglia and cerebellar circuits.
Role of ECB System in Food Intake
Its been known for a long time that cannabis consumption stimulates appetite.6 In controlled studies, exogenously administered cannabis in normal controls, induced hyperphagia, increased subjective hunger ratings,7 and increased consumption of palatable food8 and snack items.9 Greenberg et al.10 demonstrated that, on smoking D-9THC, daily caloric intake increased substantially in the first few days and then stabilized at slightly lower level. The body weight, in contrast, continued to increase progressively. On stopping the D-9 THC, both body weight and caloric intake decreased.
Animal studies have shown that CB1 antagonist but not CB2 antagonist" block D-9 THC-induced hyperphagia whereas direct injections of endocannabinoids in brain regions stimulate food consumption.12 Fasting increases levels of anandamide and 2-AG in nucleus accumbens and to lesser extent in hypothalamus, and the latter decline with feeding.13 These observations suggest a central CB1 receptor-mediated mechanism of stimulating food intake and rimonabant blocks this.
CB1 receptors are co-localized with other Systems involved in food intake, such as corticotrophin-releasing hormone (anorexigenic), and pre-pro-orexin (orexigenic). Genetic deletion of CB1 receptors increases expression of CRH,14 and Stimulation of CB1 causes sensitization of orexin-1 receptors.15
Mice with homozygous deletion of CB1 receptors (CB1-/-), when compared with wild type with intact receptors,14 show decreased percentage of fat and increased lean tissue mass; no difference in locomotor activity, thermogenesis or energy expenditure; reduction in caloric intake; and with pair-feeding, more reduction in body weight. This is suggestive of a central as well as peripheral role of ECBs in maintaining body weight.
Tolerance to Rimonabant induced hypophagia develops but with continued effects on weight loss, thereby suggestive of other mechanisms in play as well. Bensaid et al.16 demonstrated that rimonabant resulted in dose-dependent weight loss and increase in adiponectin. Adiponectin is a hormone released by fat cells and is responsible for weight loss by fatty acid oxidation, and reducing hyper-insulinaemia and Insulin resistance.17 Leptin, a hormone secreted by adipocytes, has direct regulatory action on Cocaine-amphetamine regulated transcript (CART) in hypothalamus. Reduced CART levels in CB1-/- mice14 may reflect lower levels of leptin originating from decreased body fat stores.
Recently, ECBs have been discovered to be involved in hepatic functions contributing to obesity. Osei-Hyiaman et al.18 showed that activation of CB1 receptors via administration of CB1 agonist increase lipogenesis in hepatocytes; and a high fat diet increases hepatic levels of anandamide, CB1 receptor density, and basal rates of fatty acid and CB1 blockade reduces the latter. Administration of rimonabant to obese mice results in reduced body weight, and reduced serum levels of leptin, insulin glucose, low-density lipoprotein cholesterol and triglycerides, and increased serum adiponectin.19
There is also evidence to point ECBs in reward-mediated control of food intake also. We have mentioned before about the increased intake of energy-dense food and snacks in humans. In animal experiments, CB1 agonist is self-administered in animals, which is blocked by rimonabant.20 SR141716 (now Rimonabant) was shown to selectively block the consumption of more palatable food like sugar in animals, while having no effect on regular bland food intake.21 D-9THC induced hyperphagia is reversed by naloxone22 and there appears to be synergism between opioid and endocannabinoid Systems as sub-anorectic doses of naloxone and rimonabant act synergistically to reduce feeding.6 The current view is that endocannabinoids are associated with appetitive, or incentive aspects of feeding motivation, which is related to desire to eat.3
Thus ECBs have multi-faceted role in body weight regulation including central biochemical control, peripheral energy utilization, and hedonistic evaluation of food.
Current Studies with Rimonabant
Rimonabant (ACCOMPLIA, Sanofi-Synthelabo Laboratories), is now being tested in humans for its action on obesity and potential reduction of cardiovascular risk.23 RIO-Europe was a 2-year randomized, double blind, placebo-controlled, parallel group fixed dose study. The study included patients with Body Mass Index of more than 27 kg/m2 with treated/untreated hypertension and/or dyslipidaemia; 920 patients completed first year of the study. All patients - both controls and subjects, had dietary counseling to reduce daily intake by 600 kcal. RIO-Lipids study was another multi-centre, double blind, placebo-controlled study. The study included 1036 overweight or obese patients with dyslipidaemia and body mass index between 27 to 40 kg/m2. The main findings of the study are shown in Table 1.
The results of both the studies indicate the efficacy of the compound in reducing body weight and cardiovascular risk factors. The response appears to be dose-dependent as 20 mg was in all parameters was more efficacious than 5 mg dose of rimonabant. In addition to C-reactive protein, which is an important predictor of cardiovascular risks, in a sub-group of patients the adiponectin levels were increased and leptin levels decreased as compared to baseline. In RIO-Europe study, the decrement in body weight, waist circumference and triglycerides and increase in HDL-cholesterol was sharper till 32 weeks of therapy and then became less steep. Both, active drug and placebo treated patients had diet restriction of 600 kcal per day in RIO-Europe study, but the weight loss was more pronounced in active treatment group. These findings are consistent with our current pharmacological understanding of ECB. CB1 induce food intake and lipogenesis in periphery, and these actions are blocked by rimonabant there by leading to weight loss and reduced body fat, Both these factors are important in reducing metabolic syndrome. However, eating is a complex process governed by many biochemical factors, which have been mentioned briefly above, and psychological processes. The full understanding of these factors, and their interplay with each other, will require further investigations. Also the potential role of CB1 receptors in other brain processes e.g. emotion will be blocked by rimonabant and this could limit the utility of the drug being suitable for widespread use.
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