Acetylcholine Muscarinic receptors are GPCRs of the Class A, rhodopsin-like family where the endogenous agonist is acetylcholine. There are two pharmacologically characterised allosteric sites on muscarinic receptors, one defined by it binding gallamine, strychnine and brucine, and the other defined by the binding of KT 5720, WIN 62,577, WIN 51,708 and staurosporine.