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Vol. 301, Issue 3, 1033-1041, June 2002
ek and
Division of Neuroscience Research in Psychiatry, University of
Minnesota Medical School, Minneapolis, Minnesota (E.E.E.-F.); and
Institute of Physiology, Academy of Sciences of the Czech Republic,
Prague, Czech Republic (J.J., S.T.)
Xanomeline is a potent agonist that is functionally selective for
muscarinic M1 receptors. We have shown previously that a significant fraction of xanomeline binding to membranes of Chinese hamster ovary (CHO) cells expressing the M1 receptors
occurs in a wash-resistant manner and speculated that this persistent
binding likely does not take place at the primary binding site on the receptor. In the present work we investigated in depth the
pharmacological characteristics of this unique mode of xanomeline
binding and the effects of this binding on the interaction of classical
competitive ligands with the receptor in CHO cells that express the
M1 muscarinic receptor. Onset of persistent binding of
xanomeline to the M1 muscarinic receptor was fast and was
only slightly hindered by atropine. Its dissociation was extremely
slow, with a half-life of over 30 h. Although persistently bound
xanomeline strongly inhibited binding of the classical antagonist
N-methylscopolamine (NMS) to the receptor, there are
multiple indications that this is not the result of competition at the
same binding domain. Namely, wash-resistant binding of xanomeline only
slightly slowed the rate of NMS association, but enhanced the rate of
NMS dissociation. Moreover, preincubation with xanomeline followed by
extensive washing brought about an apparent decrease in the number of
NMS binding sites. Our findings are best interpreted in terms of
allosteric interactions between xanomeline-persistent binding to the
M1 muscarinic receptor and competitive ligands bound to the
classical receptor binding site.
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