Abstract
The cannabinoid receptor one (CB1) is a class A Gβproteinβcoupled receptor thought to bind ligands primarily within its helical bundle. Evidence suggests, however, that the extracellular domain may also play a role. We have previously shown that the Cβterminus of the extracellular loop 2 of CB1 is important in binding some compounds; receptors with mutations in this region (F268W, P269A, H270A, and I271A) bound some agonists with severely reduced affinity relative to the wildβtype receptor [Ahn et al., 2009. Mol Pharmacol 76:833β842]. In the present work, we examine the impact of these mutations on binding a chemically diverse set of ligands. The receptors, F268W and I271A, exhibited a greater sensitivity to binding the inverse agonists/antagonists SLV319, AVE1625, NESS0327 relative to P269A and H270A, suggesting that the Pro and His are not involved in binding those compounds. In contrast, binding of the agonists, BAY593074 and WIN55212β2, was diminished in all four receptors, suggesting the conformational unit contributed by all four residues is important. A more marked loss in binding was observed for agonists of the nonclassical (CP55940) and classical (HUβ210, JWH061, JWH179) cannabinoid classes and for a silent antagonist derivative (Oβ2050), pointing to the critical nature of this region for binding both the bicyclic/tricyclic core and the alkyl chain of these derivatives. However, moving the location of the alkyl chain on a series of pyrazole analogues shows it can be better accommodated in certain locations (Oβ1255) than others (Oβ1302, Oβ1690) and underscores the involvement of residues F268 and I271. Drug Dev Res 71: 404β411, 2010. Β© 2010 WileyβLiss, Inc.