Muscarinic acetylcholine (mAChRs) receptors were so named because they are more sensitive to muscarine than to nicotine (Ishii M and Kurachi Y, 2006). Their counterparts are nicotinic acetylcholine receptors (nAChRs), ion channels receptors that are also important in the autonomic nervous system. Many drugs can manipulate these two distinct receptors by acting as selective agonists or antagonists. mAChRs bind to the bioamine acetylcholine, have a widespread tissue distribution and are involved in the control of numerous central and peripheral physiological responses, particularly voluntary muscle contraction. They are also major targets for drugs in human diseases such as Alzheimer's, Parkinson's and schizophrenia. This family of G-protein coupled receptors consists of five members designated M1-M5 and are sub-divided into two groups based on their primary coupling to G proteins. M2 and M4 receptors couple to Gi/o proteins and M1, M3 and M5 receptors couple to Gq/11 proteins (Caulfield MP and Birdsall NJ, 1998).