Deoxamuscaroneoxime derivatives as useful muscarinic agonists to explore the muscarinic subsite

Abstract

A series of muscarinic agonists, straight chained, branched, cyclic alkyl and aromatic derivatives of the oxime 1 (demox) was designed with the aim of investigating their activity on muscarinic receptor subtypes. Effects on M₁ receptor were assessed functionally by a microphysiometer apparatus, while M₂, M₃, and M₄ receptor potency and affinity were studied on isolated preparations of guinea pig heart, ileum, and lung, respectively. The results suggest that the substitution of a hydrogen with a long side-chain or bulky group generally induces a decrease in potency at M₁ and M₃ subtypes, while a general increase in this parameter is obtained at M₂ subtype. Among the agonists 2–18, compound 4 behaves as a full agonist with a preference for M₃ subtype. Moreover, compound 12 is inactive at M₁ and M₄ receptors while it displays a full agonist activity at M₂ and M₃ subtypes. Since demox displays a variable response on cardiac M₂ receptors regulating heart force, an in-depth inquiry of the functional behaviour of this compound was carried out at M₂ receptors. In presence of 10⁻¹¹ and 10⁻¹⁰ M demox, the binding of [³H]-NMS was increased by ≈ 30% as a consequence of an increase of the association of [³H]-NMS to membranes; this effect was not observed in presence of a higher concentration of [³H]-NMS. Higher concentrations of demox decreased the binding of [³H]-NMS to heart atrial membranes but significantly retarded the dissociation of this radioligand. Our results suggest that demox may interact with orthosteric and allosteric sites of atrial M₂ muscarinic receptor.