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.Theabsolute configuration of cannabidiol and of THC was established by correla-tion with known terpenoids [17].Several years later a minor psychotomimeti-cally active constituent, "8-THC, was isolated from marijuana [18].Whetherthis THC isomer is a natural compound, or an artifact formed during the dry-ing of the plant, remains an open problem.Several additional, non-psychotropic cannabinoids were also identified atthat time.The best known are cannabigerol [19], cannabichromene [20, 21]and cannabicyclol [22].For a better understanding of the biogenesis of acannabinoids in the plant the isolation and identification of cannabinoid acidsturned out to be essential.Alongside cannabidiolic acid, the cannabinolic andcannabigerolic acids were identified [23], followed by two "9-THC acids, Aand B [24, 25], as well as "8-THC acid [26, 27] and cannabielsoic acid [28].The decarboxylated product of cannabielsoic acid, cannabielsoin, is found inmammals as a metabolite of cannabidiol [29].The syntheses of some of thecannabinoid acids have been reported [30].A tentative pathway for the biogenesis of cannabinoids in the plant has beenpublished [31 34].However the only experimental support for "9-THC acidformation from cannabigerolic acid (by direct oxidocyclization and notthrough cannabidiolic acid as was assumed before) has been reported byShoyama s group [35].They showed that the presence of a carboxyl group inthe substrate is essential for enzymatic cyclization of the terpene moiety.Thisfinding may explain the presence of THC and THC acids in certain cannabisstrains (e.g.South African) that do not contain cannabidiol or its acid [36 38].In a series of elegant publications Shoyama s group identified an enzymeforming cannabichromenic acid and showed that this acid is formed directlyfrom cannabigerolic acid [39, 40].It is possible that some of the natural neutral cannabinoids are artifactsformed through decarboxylation, photochemical cyclization (cannabicyclol),oxidation (cannabielsoic acid) or isomerization ("8-THC and "8-THC acid) ofother constituents.Endogenous cannabinoidsThe discovery of a high-affinity, stereoselective and pharmacologically dis-tinct cannabinoid receptor in a rat brain tissue [41] led to a search for naturalendogenous ligands in the brain, which bind to this cannabinoid receptor.Weassumed that the cannabinoid receptor in the brain is not present just to bind aplant constituent, but to be activated by specific endogenous ligands.Ourapproach involved first the synthesis of a potent labeled agonist (HU-243),26 L.O.Hanua
and R.Mechoulamwhich made possible a sensitive bioassay.This compound is the most activecannabinoid known so far [65].In a standard bioassay we expected thatendogenous compounds with cannabinoid activity would displace tritiatedHU-243 bound to the central cannabinoid receptor (CB1).Rat brains are too small and hence we started our isolations with porcinebrains.After nearly 2 years of tedious work, which involved numerous chro-matographic separations, we isolated from brain an endogenous compound thatbinds to the cannabinoid receptor with about the same potency as "9-THC.Thisendogenous ligand was named anandamide [42], a name derived from theSanskrit word for bliss, ananda.When administered intraperitoneally to mice itcaused reduced activity in an immobility test and in open field tests, and pro-duced hypothermia and analgesia, a tetrad of assays typical of the psychotropiccannabinoids [43].Later we isolated two additional, apparently minor, endo-genous cannabinoids, homo-³-linoleoylethanolamide and 7,10,13,16-docosa-tetraenoylethanolamide [44].The existence of a peripheral cannabinoid receptor (CB2) led to the searchfor a ligand to this receptor.We isolated from canine gut another arachidonicacid derivative, 2-arachidonoyl glycerol (2-AG) [45].At around the same timethis compound was detected in brain [46] (see Fig.2).Hanua
et al.reported a third, ether-type endocannabinoid, 2-arachidonylglyceryl ether (noladin ether), isolated from porcine brain [47].It binds to theCB1 cannabinoid receptor (Ki = 21.2 ± 0.5 nM) and causes sedation, hypother-mia, intestinal immobility and mild antinociception in mice.It binds veryweakly to the CB2 receptor.The presence of this endocannabinoid in brain hasbeen questioned [48].However as this type of natural glycerol derivative (anether group on the 2-position) is unusual, we have repeated its isolation withan identical result (unpublished observations).In the course of the development of a bioanalytical method to assay anan-damide in brain and peripheral tissues, a compound with the same molecularweight as anandamide, but with a shorter retention time, was identified asO-arachidonoyl ethanolamine (arachidonic acid and ethanolamine joined byan ester linkage).This compound was named virodhamine [49].On the basis of previous structure activity relationship studies and on theexistence in body tissues of biosynthetic precursors, Huang et al.assumed thatN-arachidonoyl-dopamine (NADA) may exist as an endogenous capsaicin-like cannabinoid in mammalian nervous tissues and may possiblybind to the vanilloid receptor VR1 [50].They found that NADA is indeed anatural endocannabinoid in nervous tissues, with high concentrations found inthe striatum, hippocampus and cerebellum and lower concentrations in thedorsal root ganglion.NADA binds to the cannabinoid receptors with a 40-foldgreater selectivity for the CB1 (Ki = 250 ± 130 nM) than the CB2 receptor[50 52].One of the typical endocannabinoid effects is pain suppression.Someendogenous fatty acid derivatives (palmitoylethanolamide, oleamide), whichdo not bind to CB1 or CB2, either enhance this effect (the so-called entourageCannabinoid chemistry: an overview 27Figure 2.The main endocannabinoidseffect) or actually show activity by themselves, presumably by binding toas-yet unidentified cannabinoid receptors [53].Shortly after the isolation of anandamide, its biosynthesis, metabolism anddegradation in the body were studied [54, 55] [ Pobierz caÅ‚ość w formacie PDF ]