Activation of muscarinic acetylcholine receptors via their allosteric binding sites

Ligands that bind to the allosteric-binding sites on muscarinic acetylcholine receptors alter the conformation of the classical-binding sites of these receptors and either diminish or increase their affinity for muscarinic agonists and classical antagonists. It is not known whether the resulting conformational change also affects the interaction between the receptors and the G proteins. We have now found that the muscarinic receptor allosteric modulators alcuroalum, gallamine, and strychnine (acting in the absence of an agonist) alter the synthesis of cAMP in Chinese hamster ovary (CHO) cells expressing the M₂ or the M₄ subtype of muscarinic receptors in the same direction as the agonist carbachol. In addition, most of their effects on the production of inositol phosphates in CHO cells expressing the M₁ or the M₃ muscarinic receptor subtypes are also similar to (although much weaker than) those of carbachol. The agonist-like effects of the allosteric modulators are not observed in CHO cells that have not been transfected with the gene for any of the subtypes of muscarinic receptors. The effects of alcuronium on the formation of cAMP and inositol phosphates are not prevented by the classical muscarinic antagonist quinuclidinyl benzilate. These observations demonstrate for the first time that the G protein-mediated functional responses of muscarinic receptors can be evoked not only from their classical, but also from their allosteric, binding sites. This represents a new mechanism of receptor activation.