Acetylcholine, released by cholinergic neurons, activates two groups of acetylcholine receptors (AChRs); muscarinic AChRs (mAChRs), which belong to the G-protein coupled receptor (GPCR) superfamily, 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 are 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, three 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.

While most β subunits are neuronal, the β1 subunit forms functional receptors along with other subunits in the muscle³. In neurons the α2-α6 and β2-β4 subunits form heteropentameric receptors, usually with a (α_x)₂(β_y)₃ stoichiometry³.