The action of the neurotransmitter acetylcholine (ACh) is mediated through two types of receptors, the ionotrophic nicotinic receptors and the metabotrophic muscarinic receptors. The muscarinic receptors  belong to the superfamily of 7-TM G-protein-coupled receptors. Five subtypes of muscarinic receptors have been cloned and named M1-M5.^1-2

The muscarinic receptors are widely distributed throughout the body, but are predominantly expressed within the parasympathetic nervous system and exert both excitatory and inhibitory control over central and peripheral tissues.^1-2

Muscarinic receptors participate in a number of physiological functions such as regulation of heart rate, muscle contraction, cognition, sensory processing and motor control.¹ They also participate in learning and memory processing.^3-4 The M1 receptors are the most abundant muscarinic subtype in the cortex and striatum. M1 receptors were also localized in the myenteric plexus where they function as autoreceptors to enhance the release of ACh from the nerves.^5-6

The M1, M3 and M5 receptors, which are coupled to Gq/11 proteins, can protect cells from undergoing apoptosis induced by DNA damage. The signaling mechanism that mediates this anti-apoptotic response is still poorly understood. However, it was recently reported that a poly-basic motif in the C-terminus tail of the M1, M3 and M5 receptors is an essential element for the anti-apoptotic response of those receptors.⁷