Figure 1.

Schematic illustration of structure and effector systems of the mAChR subtypes. M₁, M₃ and M₅ typically couple to Ga_q and associated effector systems such as PLC. M₂ and M₄ typically couple to Ga_i/o and associated effectorsystems such as inhibition of adenylyl cyclase (AC) and ion channels, although each receptor subtype can also couple to other signaling pathways. The orthosteric binding site for ACh has been well characterized and resides in the seven transmembrane domain of the receptor. Allosteric ligands bind to other sites on the receptor to either activate (allosteric agonists) or modulate (positive and negative allosteric-site ligands) receptor function. Abbreviations: IP₃, inositol (1,4,5)-trisphosphate; ? denotes that exact allosteric binding sites have not been defined.