nAChRs are pentameric complexes made up of combinations of a number of different nAChR subunits, which can be classified as α subunits, containing two cysteine residues at positions analogous to Cys192 and Cys193, and non-alpha subunits (‘structural’ subunits), which can be defined as β subunits when they are expressed in the vertebrate nervous system. There are nine α subunits (α2–α10) and three β subunits (β2, β3, and β4) in the CNS. Nicotinic receptors are assembled as combinations of α (2-6) and and β (2-4) subunits.