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 apparent N-[3H]methylscopolamine saturation binding affinity in intact cells. Functional assays, using both M1 mAChR-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.