After the initial report by Sawzdargo et al. (1999), no more appeared in the academic literature about this orphan receptor until 2006, when attention was drawn to two patents which suggested GPR55 could bind to, and be acted upon, cannabinoid ligands (Baker et al., 2006). A GlaxoSmithKline patent described the use of yeast host strains for the coexpression of human GPR55 and chimeric yeast/human G proteins to detect ligands for the GPCR (Brown and Wise, 2001). The yeast cells are designed such that they will only propagate if there is activation of the receptor of interest and as such they can reveal both spontaneous receptor activity and, more importantly in the present context, agonist activation of the receptor (Brown et al.,
2000). This system is well suited to ligand detection for orphan receptors since the cells do not have any of their own GPCRs (Dowell and Brown, 2002). Its use showed that both rimonabant and the CB1 antagonist AM251 could activate GPR55 (Brown and Wise, 2001), thereby establishing a link with cannabinoid ligands.
A group from AstraZeneca (Drmota et al., 2004) reported the use of [3H]CP55940 and [3H]rimonabant as radiolabels for detecting ligands for GPR55 expressed in HEK293 cells. The cell membranes did not bind another cannabinoid ligand, [3H]WIN55,212-2, and therefore showed some similarity with the functional studies which suggest that WIN55,212-2 is usually not active at the rimonabant-sensitive cardiovascular sites for cannabinoids which mediate non-CB1/non-CB2 activity (White and Hiley, 1998). Surprisingly, rimonabant, which shows antagonist or inverse agonist activity in functional assays of CB1 receptors or against the non-CB1 cannabinoid effects in the cardiovascular system, was an agonist in GTPgS-binding assays using membranes containing GPR55. Other agonists in this assay included other ligands at CB1 and CB2 receptors, not only classical agonists (e.g., anandamide, 2-arachidonoylglycerol, D9-tetrahydro-cannabinol, and virodhamine) but also other the CB2 cannabinoid receptor antagonist SR144528.
It also has to be remembered that rimonabant is a relatively weak antagonist of the vascular relaxation evoked by anandamide in the rat (Kd « 0.7 mM; White and Hiley, 1997b) whereas successful binding with [3H]-rimonabant took place with 1.6 nM ligand; this implies either a very high level of receptor expression (as this concentration of rimonabant would occupy ~0.25% of the sites at the estimated Kd) or a much higher affinity for the ligand at the cloned GPR55 than at the vascular receptor. In the HEK293 cell assay, anandamide was a potent agonist (EC50 = 18.4 nM) as was D9-tetrahydrocannabinol (EC50 = 9.4 nM), while abnormal canna-bidiol was over 100 times less potent (EC5q = 2.78 mM). Therefore it might be that GPR55 is a receptor for anandamide but not the one mediating the responses to abnormal cannabidiol, which in the vascular assay is of similar potency (EC50 = 630 nM; Ho and Hiley, 2003b) to anandamide (EC50 = 650 nM; White and Hiley, 1997a).
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