Identification of WIN55212-3 as a competitive neutral antagonist of the human cannabinoid CB2 receptor

1. Several G protein-coupled receptors (GPCRs), including cannabinoid CB(1) and CB(2) 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 CB(2) receptor has been reported. 2. Here, by monitoring receptor-dependent G protein activation, we demonstrate that WIN55212-3 acts as a neutral antagonist at the human CB(2) (hCB(2)) receptor. WIN55212-3 alone, at concentrations Collapse

Key Words

• neutral antagonist
• constitutive activity
• partial inverse agonist
• endocannabinoid
• noladin ether

Collapse

MESH Headings

• Animals
• Benzoxazines
• Blotting, Western
• CHO Cells
• Cell Line
• Cerebellum/cytology
• Cerebellum/drug effects
• Cerebellum/metabolism
• Cricetinae
• Cyclohexanols/metabolism
• Dose-Response Relationship, Drug
• Guanosine 5'-O-(3-Thiotriphosphate)/metabolism
• Humans
• Isomerism
• Ligands
• Models, Molecular
• Morpholines/pharmacology
• Naphthalenes/pharmacology
• Rats
• Receptor, Cannabinoid, CB2/antagonists & inhibitors
• Receptor, Muscarinic M4/antagonists & inhibitors
• Receptors, G-Protein-Coupled/drug effects
• Receptors, Melatonin/antagonists & inhibitors
• Transfection

Collapse

Grants Collapse

Affiliation(s)

Juha R Savinainen Department of Physiology, University of Kuopio, POB 1627, FIN-70211 Kuopio, Finland.
Tarja Kokkola
Outi M H Salo
Antti Poso
Tomi Järvinen
Jarmo T Laitinen

Collapse

Cellular accumulation of anandamide: consensus and controversy

The endocannabinoids N-arachidonylethanolamine (AEA or anandamide) and 2-arachidonylglycerol (2-AG) are hypothesized to function in the brain as interneuronal signaling molecules. Prevailing models of the actions of these molecules require that they traverse cellular plasma membranes twice; first, following cellular synthesis and second, prior to enzymatic hydrolysis. The transmembrane movement of AEA has been studied in multiple laboratories with a primary focus on its cellular accumulation following extracellular administration. Although there are areas of consensus among laboratories regarding AEA accumulation, several aspects are very unclear. In particular, there is a lack of consensus in the literature regarding the importance of AEA hydrolysis by fatty acid amide hydrolase in maintaining the driving force for accumulation. Furthermore, evidence for and against a transmembrane carrier protein has been published. We have reviewed the available literature and present a working model of the processes that are involved in the cellular accumulation of AEA. It is our hypothesis that transmembrane movement of AEA is regulated by concentration gradient between extracellular and intracellular free AEA. Furthermore, it is our view that a significant portion of the intracellular AEA in most cells is sequestered either by a protein or lipid compartment and that AEA sequestered in this manner does not equilibrate directly with the extracellular pool. Finally, we discuss the available data that have been presented in support of a transmembrane carrier protein for AEA.

Despite substantial degradation, 2-arachidonoylglycerol is a potent full efficacy agonist mediating CB(1) receptor-dependent G-protein activation in rat cerebellar membranes

1. Two endocannabinoids, arachidonoyl ethanolamide (AEA) and 2-arachidonoylglycerol (2-AG) bind and activate G-protein-coupled cannabinoid receptors, but limited data exist on their relative ability to activate G-proteins. 2. Here we assess agonist potency and efficacy of various cannabinoids, including 2-AG, HU-310 (2-arachidonoyl glyceryl ether, a third putative endocannabinoid), HU-313 (another ether analogue of 2-AG), AEA, R-methanandamide (an enzymatically stable analogue of AEA), and CP-55,940 at rat brain CB(1) receptors using agonist-stimulated [(35)S]-GTPgammaS binding to cerebellar membranes and whole brain sections. Degradation of endocannabinoids under experimental conditions was monitored by HPLC. 3. To enhance efficacy differences, agonist dose-response curves were generated using increasing GDP concentrations. At 10(-6) M GDP, all compounds, except HU-313, produced full agonists responses approximately 2.5 fold over basal. The superior efficacy of 2-AG over all other compounds became evident by increasing GDP (10(-5) and 10(-4) M). 4. In membrane incubations, 2-AG was degraded by 85% whereas AEA and HU-310 were stable. Pretreatment of membranes with phenylmethylsulphonyl fluoride inhibited 2-AG degradation, resulting in 2 fold increase in agonist potency. Such pretreatment had no effect on AEA potency. 5. Responses in brain sections were otherwise consistent with membrane binding data, but 2-AG evoked only a weak signal in brain sections, apparently due to more extensive degradation. 6. These data establish that even under conditions of substantial degradation, 2-AG is a full efficacy agonist, clearly more potent than AEA, in mediating CB(1) receptor-dependent G-protein activity in native membranes.