Cannabinoid Receptors Can Activate and Inhibit G Protein-Coupled Inwardly Rectifying Potassium Channels in a Xenopus Oocyte Expression System1
- Department of Pharmacology and Toxicology, Medical College of Virginia Campus of Virginia Commonwealth University, Richmond, Virginia
Abstract
In this study, we focused on the pharmacological characterization of cannabinoid receptor coupling to G protein-gated inwardly rectifying potassium (GIRK) channels. Cannabinoids were tested on Xenopus laevis oocytes coexpressing the CB1 receptor and GIRK1 and GIRK4 channels (CB1/GIRK1/4) or the CB2 receptor and GIRK1/4 channels (CB2/GIRK1/4). WIN 55,212-2 enhanced currents carried by GIRK channels in the CB1/GIRK1/4 and CB2/GIRK1/4 system; however, the CB2 receptor did not couple efficiently to GIRK1/4 channels. In the CB1/GIRK1/4 system, WIN 55,212-2 was the most efficacious compound tested. CP 55,940 and anandamide acted as partial agonists. The rank order of potency was CP 55,940 > WIN 55,212-2 = anandamide. The CB1-selective antagonist SR141716A alone acted as a inverse agonist by inhibiting GIRK currents in oocytes expressing CB1/GIRK1/4, suggesting the CB1receptor is constitutively activated. A conserved aspartate residue, which was previously shown to be critical for G protein coupling in cannabinoid receptors, was mutated (to asparagine, D163N) and analyzed. Oocytes coexpressing CB1/GIRK1/4 or D163N/GIRK1/4 were compared. The potency of WIN 55,212-2 at the mutant receptor was similar to wild type, but its efficacy was substantially reduced. CP 55,940 did not elicit currents in oocytes expressing D163N/GIRK1/4. In summary, it appears the CB1 and CB2 receptors couple differently to GIRK1/4 channels. In the CB1/GIRK1/4 system, cannabinoids evaluated demonstrated the ability to enhance or inhibit GIRK currents. Furthermore, a conserved aspartate residue in the CB1 receptor is required for normal communication with GIRK channels in oocytes demonstrating the interaction between receptor and channels is G protein dependent.
Footnotes
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Send reprint requests to: Dr. Mary E. Abood, Forbes Norris ALS Research Center, 2351 Clay St., Suite 416, California Pacific Medical Center, San Francisco, CA 94115. E-mail:mabood{at}cooper.cpmc.org
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↵1 This work was supported by National Institutes of Health Grants DA09978 and DA05274 (to M.E.A.) and DA07027 and DA05910 (training support for S.D.M.).
- Abbreviations:
- Δ9-THC
- Δ-9-tetrahydrocannabinol
- CB1 and CB2
- cannabinoid receptor
- HP
- high potassium
- GIRK
- G protein-coupled inwardly rectifying potassium
- G protein
- guanine nucleotide binding protein
- GPCR
- G protein-coupled receptor
- IHK
- inward current with an HK solution containing 96 mM K+ and 2 mM Na+ exchanged for LK
- IAg
- inward current with application of the cannabinoid agonist WIN 55,212-2 in HK
- Inative
- native current
- CP55,940
- (−)-3-[2-hydroxyl-4-(1,1-dimethylheptyl)-phenyl]-4-[3-hydroxypropyl]cyclohexan-1-ol
- WIN 55,212-2
- (R)-(+)-[2,3-dihydro-5-methyl-3-[(4-morpholinyl)methyl]pyrolo[1,2,3-de]-1,4-benzoxazin-6-yl](1-naphthalenyl)methanone
- SR141716A
- N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamidehydrochloride
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- Received May 5, 1999.
- Accepted July 22, 1999.
- The American Society for Pharmacology and Experimental Therapeutics
