Characterization of muscarinic acetylcholine receptors from mouse brain: Evidence for regional heterogeneity and isomerization

High affinity binding of tritium labeled N-methyl-4-piperidyl benzilate to homogenates from various regions of mouse brain is used to characterize the binding mechanism to specific muscarinic sites at 25°. Binding experiments at equilibrium reveal differences in the affinity of various muscarinic agonists and antagonists for the binding sites in the various regions: cortex, putamen, hippocampus, medulla pons, and cerebellum. The density of muscarinic binding sites is also different in these regions. The regional heterogeneity is further investigated by kinetic experiments which measure the rates and the mechanisms of association and dissociation of [³H]-N-methyl-4-piperidyl benzilate. These experiments suggest that while each region contains a homogeneous population of binding sites, the association and dissociation of the ligands do not follow a simple first order mechanism. The simplest model which fits the experimental findings and which is compatible with previously published models, consists of a fast binding step followed by a slow isomerization process of the receptor-ligand complex.