Orthosteric and allosteric potentiation of heteromeric neuronal nicotinic acetylcholine receptors

Heteromeric nicotinic ACh receptors (nAChRs) were thought to have two orthodox agonist-binding sites at two alpha/beta subunit interfaces. Highly selective ligands are hard to develop by targeting orthodox agonist sites because of high sequence similarity of this binding pocket among different subunits. Recently, unorthodox ACh-binding sites have been discovered at some alpha/alpha and beta/alpha subunit interfaces, such as alpha 4/alpha 4, alpha 5/alpha 4 and beta 3/alpha 4. Targeting unorthodox sites may yield subtype-selective ligands, such as those for (alpha 4 beta 2)(2)alpha 5, (alpha 4 beta 2)(2)beta 3 and (alpha 6 beta 2)(2)beta 3 nAChRs. The unorthodox sites have unique pharmacology. Agonist binding at one unorthodox site is not sufficient to activate nAChRs, but it increases activation from the orthodox sites. NS9283, a selective agonist for the unorthodox alpha 4/alpha 4 site, was initially thought to be a positive allosteric modulator (PAM). NS9283 activates nAChRs with three engineered alpha 4/alpha 4 sites. PAMs, on the other hand, act at allosteric sites where ACh cannot bind. Known PAM sites include the ACh-homologous non-canonical site (e.g. morantel at beta/alpha), the C-terminus (e.g. Br-PBTC and 17 beta-estradiol), a transmembrane domain (e.g. LY2087101) or extracellular and transmembrane domain interfaces (e.g. NS206). Some of these PAMs, such as Br-PBTC and 17 beta-estradiol, require only one subunit to potentiate activation of nAChRs. In this review, we will discuss differences between activation from orthosteric and allosteric sites, their selective ligands and clinical implications. These studies have advanced understanding of the structure, assembly and pharmacology of heteromeric neuronal nAChRs.