Fig. 3.

(a) General structure of a single subunit of the nicotinic receptor. (b) Nicotinic receptors are formed by 5 subunits (pentamers), symmetrically arranged to delimit a pore through which cations flow, with Na⁺ and Ca²⁺ incoming, and K⁺ outgoing. Nine different types of α-subunits and 4 different types of β-subunits are known. Isoforms for subunits δ, ε, and ϒ are not yet known. Numerous receptor subtypes that have specific anatomic locations (e.g., muscle or neuronal) are generated by multiple combinations of different types of these subunits. In the central nervous system, the pentameric structures of the receptor are composed of α-subunit homodimers or of α-subunits/β-subunits combinations. (c) Top view of the structure of a neuronal nicotinic receptor composed by 2 α4-subunits (CHRNA4) and 3β2-subunits (CHRNB2). Abnormalities in this receptor, caused by mutations of its subunits, cause nocturnal frontal lobe epilepsy