Muscarinic acetylcholine receptors belong to a superfamily of seven-™-domain receptors that interact with G-proteins to initiate intracellular responses. Five muscarinic receptor subtypes have been identified and named from M1 to M5. The M3 muscarinic receptors are located at many places in the body, e.g. smooth muscles, endocrine, exocrine glands, as well as lungs. They are also found in the CNS, where it induces emesis. They generally cause smooth muscle contraction and increased glandular secretions.