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Acetylcholine, released by cholinergic neurons, activates two groups of acetylcholine receptors (AChRs); muscarinic AChRs (mAChRs) which belong to the superfamily of G-protein coupled receptors (GPCRs) and nicotinic AChRs (nAChRs) which belong to the ligand-gated ion channel superfamily. nAChRs also respond to nicotine, hence their name1.
To date, 17 different but related subunits of nAChRs have been identified and cloned. They consist of α subunits (α1-10), which is responsible for the binding of ligands. In fact, this subunit includes a Cys-loop in the first extracellular domain that is required for agonist binding2. The other subunits responsible for making up the active receptor are the β (β1-4), γ, δ and ε subunits3. Structurally, all subunits have the following: a conserved large extracellular N-terminal domain, 3 conserved transmembrane domains, a variable cytoplasmic loop and a fourth transmembrane domain with a short extracellular C-terminal domain. An active nAChR is generally a heteropentamer of these various subunits organized around a central pore1.
All α subunits are expressed in neuronal cells except for the α1 subunit which is specifically expressed in the muscle3. In the peripheral autonomic nervous system, nicotinic receptors are prominent and are responsible mediating fast synaptic transmission in all peripheral autonomic ganglia4. In this system, these channel receptors are mostly made up of two α3 subunits heteromerized with three other subunits. The α3 subunit plays a most important role in this system as knockout mice suffer from severe autonomic failure accompanied by gastrointestinal and bladder malfunctions5.
Hexamethonium is a depolarising ganglionic blocker of nicotinic nACh (NN) receptor that acts in autonomic ganglia by binding mostly in or on the NN receptor, and not the acetylcholine binding site itself. It does not have any effect on mAChR located on target organs of the parasympathetic nervous system6.
Hexamethonium bromide (#H-100) is a highly pure, synthetic, and biologically active compound.