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. In urinary bladder trachea and stomach, M2 augments the function of M3 in promoting contractility, and activation of M2 serves to counteract relaxation induced by increased cAMP levels. In addition, the ability of mChR agonists to decrease heart rate appears to be mediated primarily by M2. Agonists of mAChRs induce tremor, hypothermia, corticosterone release, and analgesia; each of these functions is mediated at least in part by M2.