Pharmacological Analysis of the Novel Mode of Interaction between Xanomeline and the M1 Muscarinic Acetylcholine Receptor

Previous findings in our laboratory suggested that the M 1 muscarinic acetylcholine receptor (mAChR) agonist xanomeline exhibits a novel mode of interaction that involves persistent binding to and activation of the M 1 mAChR, subsequent to extensive washout, as well as a possible insurmountable elem...

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Bibliographic Details
Published in:The Journal of pharmacology and experimental therapeutics 1999-06, Vol.289 (3), p.1220-1228
Main Authors: Christopoulos, A, Parsons, A M, El-Fakahany, E E
Format: Article
Language:English
Subjects:
Animals
Atropine - pharmacology
Binding, Competitive
Carbachol - pharmacology
CHO Cells
Citrulline - metabolism
Cricetinae
Humans
Kinetics
Muscarinic Agonists - pharmacology
N-Methylscopolamine - pharmacokinetics
Nitric Oxide Synthase - genetics
Nitric Oxide Synthase - metabolism
Nitric Oxide Synthase Type I
Pirenzepine - pharmacology
Pyridines - pharmacology
Radioligand Assay
Receptor, Muscarinic M1
Receptors, Muscarinic - genetics
Receptors, Muscarinic - metabolism
Receptors, Muscarinic - physiology
Recombinant Proteins - metabolism
Thiadiazoles - pharmacology
Transfection
Tritium
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Summary:Previous findings in our laboratory suggested that the M 1 muscarinic acetylcholine receptor (mAChR) agonist xanomeline exhibits a novel mode of interaction that involves persistent binding to and activation of the M 1 mAChR, subsequent to extensive washout, as well as a possible insurmountable element. In the present study, we examined this interaction in greater detail, using Chinese hamster ovary cells transfected with the genes for the M 1 mAChR and neuronal nitric oxide synthase. Pretreatment of cells with xanomeline, followed by extensive washout, resulted in elevated basal levels of neuronal nitric oxide synthase activity that were suppressed by the antagonists atropine or pirenzepine in a concentration-dependent manner. Analysis of the data yielded estimates of Schild slope factors and p K B values for the antagonists that were consistent with a model of simple competition between these latter agents and the persistently bound form of xanomeline. The ability of the antagonists to produce parallel dextral shifts of the concentration-response curves to carbachol and xanomeline was also investigated. The interaction between xanomeline and pirenzepine appeared to be insurmountable, but this may have been due to an equilibrium artifact. In contrast, the interaction between xanomeline and atropine conformed to a model of competition, indicating that the mode of interaction of free xanomeline at the M 1 mAChR is pharmacologically identical with that of the persistently bound form. Radioligand binding studies also showed that the presence of various concentrations of xanomeline had no significant effect on the calculated affinity of atropine or pirenzepine in inhibiting the binding of [ 3 H] N -methylscopolamine. Overall, these findings suggest that the persistent attachment of xanomeline to the M 1 mAChR does not prevent this agonist from interacting with the classic binding site in a competitive fashion.
ISSN:0022-3565
1521-0103