Abstract
We investigated the molecular nature of the interaction between the functionally selective M1 muscarinic acetylcholine receptor (mAChR) agonist xanomeline and the human M1 mAChR expressed in Chinese hamster ovary (CHO) cells. In contrast to the non-subtype-selective agonist carbachol, xanomeline demonstrated M1 mAChR binding that was resistant to extensive washout, resulting in a significant reduction in apparentN-[3H]methylscopolamine saturation binding affinity in intact cells. Functional assays, using both M1mAChR-mediated phosphoinositide hydrolysis and activation of neuronal nitric oxide synthase, confirmed that this persistent binding resulted in elevated basal levels of system activity. Furthermore, this phenomenon could be reversed by the addition of the antagonist atropine. However, pharmacological analysis of the inhibition by atropine of xanomeline-mediated functional responses indicated a possible element of noncompetitive behavior that was not evident in several kinetic and equilibrium binding experimental paradigms. Taken together, our findings indicate for the first time a novel mode of interaction between an mAChR agonist and the M1 mAChR, which may involve unusually avid binding of xanomeline to the receptor. This yields a fraction of added agonist that is retained at the level of the receptor compartment to persistently bind to and activate the receptor subsequent to washout. The results of the current study suggest that elucidation of the mechanism or mechanisms of interaction of xanomeline with the M1 mAChR is particularly important in relation to the potential therapeutic use of this agent in the treatment of Alzheimer’s disease.
Footnotes
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Send reprint requests to: Prof. Esam E. El-Fakahany, Division of Neuroscience Research in Psychiatry, Box 392, Mayo Memorial, University of Minnesota Medical School, Minneapolis, MN 55455. E-mail: elfak001{at}maroon.tc.umn.edu
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This work was supported by National Institutes of Health Grant NS25743.
- Abbreviations:
- mAChR
- muscarinic acetylcholine receptor
- CHO
- Chinese hamster ovary
- CCh
- carbachol
- NMS
- N-methylscopolamine
- HEPES
- 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid
- PI
- phosphoinositide
- nNOS
- neuronal nitric oxide synthase
- DMEM
- Dulbecco’s modified Eagle’s medium
- dpm
- disintegrations per minute
- ANOVA
- analysis of variance
- Received November 14, 1997.
- Accepted February 23, 1998.
- The American Society for Pharmacology and Experimental Therapeutics
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