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
- Received May 5, 1999.
- Accepted July 22, 1999.
- The American Society for Pharmacology and Experimental Therapeutics
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Cannabinoid Receptors Can Activate and Inhibit G Protein-Coupled Inwardly Rectifying Potassium Channels in a Xenopus Oocyte Expression System
