The action of the neurotransmitter acetylcholine 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 exerts 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