The muscarinic acetylcholine receptor family consists of five GPCRs that mediate some of the neurotransmission functions of acetylcholine in the CNS and the periphery. The M5 receptor, along with the M1 and M3 receptors, signal through Gq/11 and subsequent release of Ca2+ from the ER. Although M5 is expressed in the CNS at relatively low levels, it appears to be the only muscarinic acetylcholine receptor expressed in midbrain dopaminergic neurons, and it has been shown to mediate dopamine release from these neurons. The M5 receptor is also expressed in blood vessels in the brain and the periphery, and studies with M5 knockout mice demonstrate that M5 mediates the vasodilatory action of acetylcholine on cerebral microvessels. Further studies with M5 knockout mice also indicate a role for M5 in the rewarding effects of cocaine and morphine.