The M1 receptor (Peralta EG et al, 1987) is found in exocrine glands and the CNS. It mediates slow excitatory postsynaptic potential (EPSP) at the ganglion in the postganglionic nerve. The M3 receptor (Peralta EG et al, 1987) is found in smooth muscle of the blood vessels and in the lungs. M3 mediates vascular relaxation (by activating vascular endothelial cells causing increased NO synthesis) and lung constriction (by coupling to Gq protein causing increased intracellular calcium). The location of the M5 receptor is not well known but thought to be in the CNS.
All of these three receptors couple with Gq/11 protein which use the upregulation of phospholipase C and therefore inositol trisphosphate and intracellular calcium as a signaling mechanism (Bräuner-Osborne H and Brann MR, 1996).