Cholinergic Receptors: Muscarinic

Muscarinic receptors are classified into five subtypes: M1 to M5. Structurally, all are G protein-coupled receptors (GPCRs). The odd-numbered subtypes (M1, M3, M5) couple with Gq proteins, while M2 and M4 couple with Gi proteins. This structural difference determines their distinct signaling pathways and physiological effects.

M1, M3, and M5 receptors couple to Gq proteins that activate phospholipase C, triggering the IP3/DAG pathway. This increases intracellular calcium mobilization from the endoplasmic reticulum. The released calcium and DAG activate protein kinase C, which phosphorylates proteins to generate physiological responses.

M2 and M4 receptors couple to Gi proteins that inhibit adenylyl cyclase, decreasing cAMP levels. This opens inwardly rectifying potassium channels, causing hyperpolarization of excitable membranes. Additionally, these receptors inhibit voltage-gated calcium channels, further suppressing neuronal and cardiac excitability and regulating overall tissue responsiveness.

M1 receptors, also called neural receptors, are located in the central nervous system, peripheral nervous system, and parietal cells. They regulate gastrointestinal secretions following vagal stimulation, learning, motor function, and memory. These receptors are essential for cognitive and autonomic functions.

M2 receptors, called cardiac receptors, are located primarily in heart tissue where they regulate negative inotropic and chronotropic effects while stimulating smooth muscle contractions. M3 receptors are abundant in smooth muscles and glands, where their activation stimulates muscle contraction and gland secretion through direct acting cholinergic agonists pharmacological actions.

Muscarinic receptors have greater affinity for the alkaloid muscarine than for acetylcholine or nicotine, which is the basis for their classification. Nicotinic receptors, by contrast, have higher affinity for nicotine. This differential affinity allows selective pharmacological targeting of each receptor family.

M4 and M5 receptors are located mainly in the central nervous system where they limit neurotransmitter release. These receptors modulate synaptic transmission by inhibiting the release of various neurotransmitters, contributing to overall regulation of neural signaling, cognitive processes, and behavioral responses.