Muscarinic acetylcholine receptors are members of the superfamily of G protein-coupled receptors (GPCRs). They are relatively abundant and mediate many of the diverse actions of acetylcholine in the CNS as well as throughout non-nervous tissues innervated by the parasympathetic nervous system. At present, five receptor subtypes (M1 - M5) have been identified. The odd-numbered receptors (M1, M3, and M5) are preferentially coupled to G(q/11). The even-numbered receptors (M2 and M4) are coupled to G(i/o). They also activate G protein-gated potassium channels, which leads to hyperpolarization of the plasma membrane in different excitable cells. Among the five subtypes, M3 receptor is important in smooth muscle contraction, increased endocrine and exocrine gland secretion, vasodilation, eye accommodation, emesis and other CNS effects.