Cholinergic receptors or cholinoceptors are classified into muscarinic or nicotinic receptors based on their affinity for a particular alkaloid—muscarine or nicotine. The mAChR have more affinity for muscarine than ACh or nicotine. Structurally, they are GPCRs and are classified into five subtypes: M1 to M5 . The odd-numbered subtypes are linked with a Gq protein that activates phospholipase C via the IP3 /DAG pathway which increases intracellular Ca2+ to generate physiological responses. On the other hand the M2 and M4 receptors are coupled to Gi proteins and inhibit adenylyl cyclase. This decreases cAMP and opens K+ channels resulting in hyperpolarization of excitable tissues. M1 receptors are located in the CNS, exocrine glands, and autonomic ganglia, while M2 receptors are located in the cardiac tissues. M3 receptors are located in exocrine glands and smooth muscles. M4 and M5 receptors are located mainly in the CNS.