The muscarinic acetylcholine receptor (mAChR) family consists of five GPCRs that mediate some of the neurotransmission functions of acetylcholine in the CNS and the periphery. The M1, M3 and M5 receptors couple to Gq to mobilize intracellular calcium, whereas the M2 and M4 receptors couple to Gi/o to inhibit cAMP production. M3 is expressed prominently in smooth muscle, and plays a primary role in mediating mAChR agonist-induced contractility. Mice lacking M3 have dilated pupils, which indicates a role for M3 in regulating tone of the pupillary sphincter muscle. In addition, M3 plays a role in feeding, as indicated by the lean and hypophagic phenotype of M3-null mice.