Radioligand Binding at Muscarinic Receptors

Five subtypes of muscarinic acetylcholine receptors denoted M1 through M5 mediate a wide array of physiological functions. Therefore, impairment of muscarinic signaling is involved in numerous diseases and pathological conditions including Alzheimer's disease, schizophrenia, and Parkinson’s disease. Muscarinic receptors are also pharmacological targets for the treatment of asthma and chronic obstructive pulmonary disease. Radioligand binding techniques are orthodox but allow a very precise study of the involvement of individual muscarinic receptor subtypes in the physiology and pathology of muscarinic signaling, the study of the structure of muscarinic receptors and the structure-activation relationship of muscarinic ligands. Here we discuss the current state of knowledge of radioligand binding experiments at muscarinic receptors from the perspective of available radioligands and selective unlabeled muscarinic ligands. We relate the binding properties of muscarinic ligands to experimental design (e.g., non-specific binding determination, incubation conditions, buffers, temperature, etc.). We also list tissue/cell sources of muscarinic receptors suitable for radioligand binding studies and describe procedures of cell and tissue preparation for radioligand binding experiments. We also discuss several techniques of receptor-bound ligand separation applicable to muscarinic receptors and provide basic information for binding data analysis.