The quinuclidinol nucleus appears as a common pharmacophore in many ligands of muscarinic receptors (mAChR), of which there are five subtypes named M1-5. These are G protein-coupled acetylcholine receptors found in the plasma membranes of certain neurons and other cells, and are involved in diverse physiological functions such as smooth muscle contraction, exocrine and endocrine secretion, and nerve-to-nerve transmision. Radiolabelled derivatives of 3-quinuclidinyl benzilate (1), a non subtype-selective muscarinic receptor antagonist, are used in receptor binding assays and research into the roles of the individual receptors.