The endocannabinoid system consists of G protein-coupled cannabinoid CB(1) and CB(2) receptors, of endogenous compounds known as endocannabinoids that can target these receptors, of enzymes that catalyse endocannabinoid biosynthesis and metabolism, and of processes responsible for the cellular uptake of some endocannabinoids. This review presents in vitro evidence that most or all of the following 13 compounds are probably orthosteric endocannabinoids since they have all been detected in mammalian tissues in one or more investigation, and all been found to bind to cannabinoid receptors, probably to an orthosteric site: anandamide, 2-arachidonoylglycerol, noladin ether, dihomo-γ-linolenoylethanolamide, virodhamine, oleamide, docosahexaenoylethanolamide, eicosapentaenoylethanolamide, sphingosine, docosatetraenoylethanolamide, N-arachidonoyldopamine, N-oleoyldopamine and haemopressin. In addition, this review describes in vitro findings that suggest that the first eight of these compounds can activate CB(1) and sometimes also CB(2) receptors and that another two of these compounds are CB(1) receptor antagonists (sphingosine) or antagonists/inverse agonists (haemopressin). Evidence for the existence of at least three allosteric endocannabinoids is also presented. These endogenous compounds appear to target allosteric sites on cannabinoid receptors in vitro, either as negative allosteric modulators of the CB1 receptor (pepcan-12 and pregnenolone) or as positive allosteric modulators of this receptor (lipoxin A(4)) or of the CB(2) receptor (pepcan-12). Also discussed are current in vitro data that indicate the extent to which some established or putative orthosteric endocannabinoids seem to target non-cannabinoid receptors and ion channels, particularly at concentrations at which they have been found to interact with CB(1) or CB(2) receptors.
Keywords: 2-Arachidonoylglycerol; Anandamide; Cannabinoid receptors; Dihomo-γ-linolenoylethanolamide; Docosahexaenoylethanolamide; Docosatetraenoylethanolamide; Eicosapentaenoylethanolamide; Endocannabinoid pharmacology; Haemopressin; Lipoxin A4; N-arachidonoyldopamine; N-oleoyldopamine; Noladin ether; Oleamide; Pepcan-12; Pregnenolone; Sphingosine; Virodhamine.