Cannabinoid Receptor Ligands Roger G Pertwee School of Medical Sciences, Institute of Medical Sciences, University of Aberdeen, Foresterhill, Aberdeen AB25 2ZD, Scotland, UK. Phone: + 44 (0)1224 555740, Fax: + 44 (0)1224 555844, E-mail: [email protected] Roger Pertwee is currently Professor of Neuropharmacology at the University of Aberdeen and Director of Pharmacology for GW Pharmaceuticals. His research focuses on the pharmacology of cannabis and its constituents and of cannabinoid receptors and cannabis-derived, synthetic and endogenous ligands for these receptors. The Endocannabinoid System including immune cells. CB2 receptors are located predominantly in immune cells both within and Two types of cannabinoid receptor have so far been outside the central nervous system, the functions identified.1,2 These are the CB receptor, cloned 1 of these receptors including modulation of cytokine in 1990,3 and the CB receptor, cloned in 1993,4 2 release and of immune cell migration. In the brain, both of which are members of the superfamily of CB receptors are expressed by microglia,7 by blood G-protein-coupled receptors. The cloning of these 2 vessels,7 and by some neurons.8,9 However, the role receptors prompted the development of mice from of neuronal CB2 receptors is currently unknown. which cannabinoid CB1 and/or CB2 receptors have been genetically deleted and these transgenic The central distribution pattern of CB1 receptors is animals, particularly CB1 knockout mice, are now heterogeneous and accounts for several prominent widely used to explore the physiological and pharmacological properties of CB1 receptor agonists, pathological functions of cannabinoid receptors.1,5,6 for example their ability to impair cognition and CB1 receptors are found mainly at the terminals of memory and to alter the control of motor function. Thus central and peripheral neurons where they usually the cerebral cortex, hippocampus, lateral caudate- mediate inhibition of neurotransmitter release. putamen, substantia nigra pars reticulata, globus They are also present in some non-neuronal cells, pallidus, entopeduncular nucleus and the molecular Figure 1 | Structures of the plant cannabinoids, ∆9-tetrahydrocannabinol (∆9-THC), ∆8-tetrahydrocannabinol (∆8-THC), the synthetic cannabinoids HU 210, CP 55,940 , WIN 55,212-2 and WIN 55,212-3, and the endogenous cannabinoids anandamide and 2-arachidonylglycerol (see also Table 1) O O 11 9 N N 8 10 OH O O H 1 OH 7 10b H 2 10a N N H 3 13 H 6 O 5 12 4 1' 3' 5' O O O ∆9-THC ∆8-THC WIN 55,212-2 (Cat. No. 1038) WIN 55,212-3 (Cat. No. 2327) OH HO OH OH O H OH 8 5 3 1' O OH N H H 2' OH 10 O O 20 OH 17 19 HU 210 (Cat. No. 0966) CP 55,940 (Cat. No. 0949) Anandamide (Cat. No. 1339) 2-Arachidonylglycerol (Cat. No. 1298) (Bold Text Denotes Compounds Available From Tocris) Tocris Bioscience Scientific Review Series Tocris Bioscience Scientific Review Series layer of the cerebellum are all populated with such endogenous cannabinoids (endocannabinoids) 1,10 particularly high concentrations of CB1 receptors. to be identified were N-arachidonoyl ethanolamine In line with the analgesic properties of cannabinoid (anandamide) in 1992 and 2-arachidonylglycerol in 20-22 receptor agonists, CB1 receptors are also found on 1995 (Figure 1), both of which are synthesised pain pathways in the brain and spinal cord and at the on demand in response to elevations of intracellular peripheral terminals of primary sensory neurons.11,12 calcium.23 Anandamide is formed from N-arachidonoyl Although the concentration of CB1 receptors is phosphatidylethanolamine in a process that is considerably less in peripheral tissues than in the catalysed by N-acyl phosphatidylethanolamine- central nervous system, this does not mean that selective phospholipase D (NAPE-PLD). The peripheral CB1 receptors are unimportant. Thus in synthesis of 2-arachidonylglycerol, however, some peripheral tissues, discrete regions such as is thought to depend on the conversion of nerve terminals that form only a small part of the 2-arachidonate-containing phosphoinositides to total tissue mass are known to be densely populated diacylglycerols and on their subsequent with CB1 receptors. Peripheral tissues in which transformation to 2-arachidonylglycerol by the CB1 receptors are expressed on neurons include action of two diacylglycerol lipase (DAGL) isozymes, the heart, vas deferens, urinary bladder and small DAGLα and DAGLβ.23,24 Following their synthesis and intestine.10,13 release, these endocannabinoids are removed from their sites of action by cellular uptake and degraded Both CB1 and CB2 receptors are coupled through by enzymes, 2-arachidonylglycerol mainly by Gi/o proteins, negatively to adenylyl cyclase and monoacylglycerol lipase (MAGL) but also by fatty acid 1,14 positively to mitogen-activated protein kinase. In amide hydrolase (FAAH), and anandamide by FAAH addition, CB1 receptors are coupled to ion channels and/or by palmitoylethanolamide-preferring acid through Gi/o proteins, positively to A-type and inwardly amidase (PAA), cyclooxygenase-2, lipoxygenases rectifying potassium channels and negatively to and cytochrome P450.5,23-25 Other ligands that may 1,13,14 N-type and P/Q-type calcium channels. CB1 be endocannabinoids are 2-arachidonylglyceryl receptors can also couple to Gs proteins to activate ether (noladin ether), O-arachidonoyl ethanolamine 15-17 adenylyl cyclase, the extent to which this occurs (virodhamine), N-dihomo-γ-linolenoyl ethanolamine, possibly being determined by the location of these N-docosatetraenoyl ethanolamine, oleamide, receptors or by cross-talk between CB1 receptors N-arachidonoyl dopamine (NADA) and and co-localised G-protein-coupled non-CB1 N-oleoyl dopamine (OLDA) (Figures 2 and 3).5 15,16,18,19 receptors. It may also be that CB1 receptors Endocannabinoids together with their receptors can exist as two distinct subpopulations, one constitute what is now usually referred to as the 15 coupled to Gi/o proteins and the other to Gs. Details ‘endocannabinoid system’. of additional signalling mechanisms that have been While it is generally accepted that endocannabinoids proposed for cannabinoid CB1 and CB2 receptors can be found elsewhere.1,14 do pass through cell membranes, one issue that is currently very much a matter of debate is the question The cloning of cannabinoid receptors was followed of whether the cellular uptake of endocannabinoids by the discovery that mammalian tissues produce such as anandamide is mediated by a transporter.25-27 compounds that can activate these receptors. The first In contrast, FAAH is now well characterised. Indeed, Figure 2 | Structures of the endogenous cannabinoid receptor agonists, N-dihomo-γ-linolenoyl ethanolamine, N-docosatetraenoyl ethanolamine, NADA, oleamide, OLDA and virodhamine OH O O O OH OH N N N OH H H H N-Dihomo-γ-linolenoyl ethanolamine N-Docosatetraenoyl ethanolamine NADA (Cat. No. 1568) OH O O O NH2 NH2 N OH O H Oleamide (Cat. No. 0878) OLDA (Cat. No. 1641) Virodhamine (Cat. No. 1569) (Bold Text Denotes Compounds Available From Tocris) | Cannabinoid Receptor Ligands it has been cloned28 and FAAH knockout mice mechanism(s) underlying the entourage effect have have been developed.29,30 NAPE/PLD,31 MAGL,32-34 yet to be established. and DAGLα and DAGLβ35 have also been cloned, Endocannabinoids most probably have both and mice with a genetic deletion of NAPE/PLD neuromodulatory and immunomodulatory roles that generated.36 include inhibition of ongoing transmitter release At least some effects induced by endogenously through retrograde signalling38 and regulation of released anandamide and 2-arachidonylglycerol cytokine release and of immune cell migration.39,40 appear to be enhanced through what has been It is also now generally accepted that there are termed the “entourage effect”. This relies on certain disorders in which endocannabinoid release the co-release of other endogenous fatty acid increases in particular tissues, and secondly, that this derivatives that include palmitoylethanolamide upregulation of the endocannabinoid system leads and oleamide, which can potentiate anandamide, in some instances to the suppression of unwanted and 2-linoleoylglycerol and 2-palmitoylglycerol, signs and symptoms and so is “autoprotective” and in which can potentiate 2-arachidonylglycerol.37 The others to the production of undesirable effects.5 Thus Table 1 | Pharmacological properties of certain cannabinoid CB1/CB2 receptor agonists and their Ki 3 3 values for the in vitro displacement of [ H]CP 55,940 or [ H]HU 243 from CB1- and CB2-specific binding sites CB1 Ki CB2 Ki Classification Examples values values (nM) (nM) Classical The compounds in this group consist of dibenzopyran derivatives and are either plant-derived cannabinoids or synthetic analogues of these. Notable examples are 9 • (–)-∆ -THC, which binds equally well to CB1 and CB2 receptors and behaves as a partial agonist 5.05 to 3.13 to at both of these receptor types. It has even less efficacy at CB2 than at CB1 receptors and, 80.3 75.3 42 indeed, has been reported in one CB2 bioassay system to behave as an antagonist. 8 9 • (–)-∆ -THC, which resembles ∆ -THC both in its affinities for CB1 and CB2 receptors and in its 44, 44, CB1 receptor efficacy. 47.6 39.3 8 • (–)-11-hydroxy-∆ -THC-dimethylheptyl (HU 210), which has efficacies at CB1 and CB2 receptors 0.06 to 0.17 to that match those of CP 55,940 and WIN 55,212-2 (see below) and affinities for CB1 and CB2 0.73 0.52 receptors that exceed those of many other cannabinoids. It is a particularly potent cannabinoid receptor agonist and its pharmacological effects in vivo are exceptionally long-lasting. The enhanced affinity and efficacy shown by HU 210 at cannabinoid receptors can be largely attributed to the replacement of the pentyl side chain of ∆8-THC with a dimethylheptyl group. Nonclassical The compounds in this group were developed by a Pfizer research team.