Differential regulation of muscarinic M1 receptors by orthosteric and allosteric ligands.
BACKGROUND
Activation of muscarinic M1 receptors is mediated via interaction of orthosteric agonists with the acetylcholine binding site or via interaction of allosteric agonists with different site(s) on the receptor. The focus of the present study was to determine if M1 receptors activated by allosteric agonists undergo the same regulatory fate as M1 receptors activated by orthosteric agonists.


RESULTS
The orthosteric agonists carbachol, oxotremorine-M and pilocarpine were compared to the allosteric agonists AC-42, AC-260584, N-desmethylclozapine and xanomeline. All ligands activated M1 receptors and stimulated interaction of the receptors with beta-arrestin-1. All ligands reduced cell surface binding and induced the loss of total receptor binding. Receptor internalization was blocked by treatment with hypertonic sucrose indicating that all ligands induced formation of clathrin coated vesicles. However, internalized receptors recycled to the cell surface following removal of orthosteric, but not allosteric agonists. Whereas all ligands induced loss of cell surface receptor binding, no intracellular vesicles could be observed after treatment with AC-260584 or xanomeline. Brief stimulation of M1 receptors with AC-260584 or xanomeline resulted in persistent activation of M1 receptors, suggesting that continual receptor signaling might impede or delay receptor endocytosis into intracellular vesicles.


CONCLUSIONS
These results indicate that allosteric agonists differ from orthosteric ligands and among each other in their ability to induce different regulatory pathways. Thus, signaling and regulatory pathways induced by different allosteric ligands are ligand specific.
Davis
Christopher N
CN
ACADIA Pharmaceuticals Inc, 3911 Sorrento Valley Blvd, San Diego, CA 92121, USA. davis04@gmail.com
Bradley
Stefania Risso
SR
Schiffer
Hans H
HH
Friberg
Mikael
M
Koch
Kristian
K
Tolf
Bo-Ragnar
BR
Bonhaus
Douglas W
DW
Lameh
Jelveh
J
D000494:Allosteric Regulation / Q000187:drug effects / Q000502:physiology; D000818:Animals; D016466:CHO Cells; D002460:Cell Line; D006224:Cricetinae; D003412:Cricetulus; D006801:Humans; D008024:Ligands; D018721:Muscarinic Agonists / Q000737:chemistry* / Q000378:metabolism* / Q000494:pharmacology; D011485:Protein Binding / Q000187:drug effects / Q000502:physiology; D043643:Receptor, Muscarinic M1 / Q000819:agonists* / Q000378:metabolism* / Q000502:physiology
D008024:Ligands; D018721:Muscarinic Agonists; D043643:Receptor, Muscarinic M1