Abstract

Distinct pharmacological phenotypes of muscarinic acetylcholine receptors (mAChRs) have been proposed. We compared the pharmacological profiles of mAChRs in intact segments and homogenates of rat cerebral cortex and other tissues by using radioligand binding assays with [³H]N-methylscopolamine ([³H]NMS). Recombinant M₁ and M₃ mAChRs were also examined. The density of mAChRs detected by [³H]NMS binding to rat cerebral cortex segments and homogenates was the same (approximately 1400 fmol/mg tissue protein), but the dissociation constant of [³H]NMS was significantly different (1400–1700 pM in segments and 260 pM in homogenates). A wide variation in [³H]NMS binding affinity was also observed among the segments of other tissues (ranging from 139 pM in urinary bladder muscle to 1130 pM in the hippocampus). The mAChRs of cerebral cortex were composed of M₁, M₂, M₃, and M₄ subtypes, which showed typical subtype pharmacology in the homogenates. However, in the cortex segments the M₃ subtype showed a low selectivity for M₃ antagonists (darifenacin, solifenacin) and was not distinguished by the M₃ antagonists from the other subtypes. Recombinant M₁ and M₃ mAChRs showed high affinity for [³H]NMS and subtype-specific pharmacology for each tested ligand. The present binding study under conditions where tissue integrity was kept demonstrates a wide variation in [³H]NMS binding affinity among mAChRs of many rat tissues and the presence of an atypical M₃ phenotype in the cerebral cortex, suggesting that the pharmacological properties of mAChRs are not necessarily constant, rather they may be significantly modified by tissue integrity and tissue type.