Article date: July 2005
By: Juha R Savinainen, Tarja Kokkola, Outi M H Salo, Antti Poso, Tomi Järvinen, Jarmo T Laitinen in Volume 145, Issue 5, pages 636-645
Several G protein‐coupled receptors (GPCRs), including cannabinoid CB1 and CB2 receptors, show constitutive activity under heterologous expression. Such a tonic response is generated in the absence of an activating ligand, and can be inhibited by inverse agonists. Neutral antagonists, however, are silent at such receptors, but can reverse both agonist and inverse agonist responses. To date, no neutral antagonist for the CB2 receptor has been reported.
Here, by monitoring receptor‐dependent G protein activation, we demonstrate that WIN55212‐3 acts as a neutral antagonist at the human CB2 (hCB2) receptor. WIN55212‐3 alone, at concentrations 10−4 M, behaved as a silent ligand exhibiting no agonist or inverse agonist activity. However, WIN55212‐3 competitively antagonized cannabinoid agonist CP‐55,940‐stimulated responses (pA2 6.1). Importantly, the inverse agonism evoked by SR144528 in hCB2 was dose‐dependently reversed by WIN55212‐3 (pEC50 5.3±0.2), indicating true neutral antagonist behavior.
Furthermore, WIN55212‐3 also antagonized CB1 receptor signaling in a competitive manner (pA2 5.6), but behaved as a partial inverse agonist (pIC50 5.5±0.1) at the constitutively active human CB1.
Additionally, WIN55212‐3 antagonized signaling of the human melatonin MT1 receptor, with modest activity at the human muscarinic M4 receptor, but it was inactive towards several other GPCRs.
These data identify WIN55212‐3 as a true neutral hCB2 receptor antagonist. WIN55212‐3 offers a valuable tool for further characterization of ligand activities at the CB2 receptor and may serve as a lead compound in further efforts to develop more potent and selective neutral CB2 receptor antagonists.
Several G protein‐coupled receptors (GPCRs), including cannabinoid CB1 and CB2 receptors, show constitutive activity under heterologous expression. Such a tonic response is generated in the absence of an activating ligand, and can be inhibited by inverse agonists. Neutral antagonists, however, are silent at such receptors, but can reverse both agonist and inverse agonist responses. To date, no neutral antagonist for the CB2 receptor has been reported.
Here, by monitoring receptor‐dependent G protein activation, we demonstrate that WIN55212‐3 acts as a neutral antagonist at the human CB2 (hCB2) receptor. WIN55212‐3 alone, at concentrations 10−4 M, behaved as a silent ligand exhibiting no agonist or inverse agonist activity. However, WIN55212‐3 competitively antagonized cannabinoid agonist CP‐55,940‐stimulated responses (pA2 6.1). Importantly, the inverse agonism evoked by SR144528 in hCB2 was dose‐dependently reversed by WIN55212‐3 (pEC50 5.3±0.2), indicating true neutral antagonist behavior.
Furthermore, WIN55212‐3 also antagonized CB1 receptor signaling in a competitive manner (pA2 5.6), but behaved as a partial inverse agonist (pIC50 5.5±0.1) at the constitutively active human CB1.
Additionally, WIN55212‐3 antagonized signaling of the human melatonin MT1 receptor, with modest activity at the human muscarinic M4 receptor, but it was inactive towards several other GPCRs.
These data identify WIN55212‐3 as a true neutral hCB2 receptor antagonist. WIN55212‐3 offers a valuable tool for further characterization of ligand activities at the CB2 receptor and may serve as a lead compound in further efforts to develop more potent and selective neutral CB2 receptor antagonists.
British Journal of Pharmacology (2005) 145, 636–645. doi:10.1038/sj.bjp.0706230
DOI: 10.1038/sj.bjp.0706230
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