Cannabinoid Receptors And Endocannabinoids

The previous review article in this series described the discovery and cloning of cannabinoid CB1 and CB2 receptors and a classification of these receptors was provided in 2002 [1, 6]. Cannabinoid CB1 receptors are expressed primarily within the central nervous system (CNS), where they are widely distributed. The CB1 receptor is also present in some peripheral tissues. The CB2 receptor is found mainly in cells of the immune system and exhibits 48% homology with the CB1 receptor. Both receptors are coupled to G;/o proteins. Evidence exists for the presence of additional cannabinoid receptor subtypes, as covered by a recent review article [8]. Furthermore, a recent patent application described high-affinity binding of a number of cannabinoid ligands to GPR55, suggesting that this might be one of the additional cannabinoid receptors responsible for the pharmacological observations [9].

Since the cloning of the cannabinoid receptors, their endogenous ligands, the endocannabinoids, have received a great deal of research interest. A number of recent review articles have extensively covered the end-ocannabinoid system [10-12], so the coverage in this article will be brief.

The best studied of the endocannabinoids are anandamide (N-arachidonyl-ethanolamine, AEA)(1) and 2-arachidonylglycerol (2-AG)(2). Anandamide was first identified from porcine brain extracts by Devane and co-workers in 1992 [13], while 2-AG was first reported in 1995 to have been isolated from canine gut [14] and rat brain [15]. More recently, noladin ether (2-arachidonyl-glyceryl ether, 2-AGE)(3) [16], virodhamine (O-arachidonyl-ethanolamine)(4) [17] and N-arachidonyl-dopamine (NADA)(5) [18] were proposed as endogenous ligands for the cannabinoid receptors. In a subsequent publication, the authors failed to detect noladin ether in mammalian brains and questioned the relevance of this compound as an endocannabinoid [19]. Anandamide, noladin ether and NADA have functional selectivity for CB1 receptors, virodhamine is CB2 selective and 2-AG is essentially non-selective.

(1) Anandamide R = CONH(CH2)2OH

(2) 2-Arachidonylglycerol R = CO2CH(CH2OH)2 >(CH2)3R (3) Noladin ether R = CH2OCH(CH2OH)2

(4) Virodhamine R = CO2(CH2)2NH2

(5) N-Arachidonyl-dopamine R = CONH(CH2)2-3,4-(OH)2-Ph n^HL

The biosyntheses of anandamide and 2-AG have been studied in depth [10]. These compounds appear to be synthesised on demand in response to certain stimuli, rather than being stored in cells. Little is known regarding the biosynthesis of noladin ether, virodhamine or NADA.

The mechanism of release of endocannabinoids from cells and their subsequent re-uptake remain as points of discussion. Several groups have proposed the existence of an anandamide transporter that can selectively release and remove endocannabinoids from their site of action [20-22]. Indeed, a number of compounds have been proposed as inhibitors of the putative anandamide transporter as will be outlined in the following section. An alternative suggestion is that the release and re-uptake of endocannabinoids is a simple diffusion process, with concentrations and hence diffusion being driven by the enzyme fatty acid amide hydrolase (FAAH) [23]. This suggestion has recently been refuted by Fegley and co-workers [24], who investigated anandamide internalisation and activity in wild-type and FAAH knock-out mice treated with anandamide transport (ANT) inhibitors, concluding that anandamide uptake was independent of FAAH activity. The discussions will only be concluded once the putative anandamide transporter is identified and cloned.

FAAH was originally purified and cloned from rat liver microsomes and is able to catalyse the hydrolysis of anandamide and 2-AG, in addition to other long-chain fatty acid amides [25]. Studies into the structure and role of this enzyme have generated interest in the potential therapeutic applications of FAAH inhibitors [26-28]. FAAH knock-out mouse brains contained 15fold higher levels of anandamide than their wild-type counterparts and these animals have also been shown to be more responsive to exogenously administered anandamide [29]. These animals also showed a reduced response to painful stimuli, supporting the hypothesis that FAAH inhibition may provide novel analgesics. Levels of 2-AG were not elevated in the FAAH knock-out animals, apparently due to the existence of alternative metabolic fates for this compound [30].

0 0

Post a comment