Selective Ligand Behaviors Provide New Insights into Agonist Activation of Nicotinic Acetylcholine Receptors

Abstract

Nicotinic acetylcholine receptors are a diverse set of ion channels that are essential to everyday brain function. Contemporary research studies selective activation of individual subtypes of receptors, with the hope of increasing our understanding of behavioral responses and neurodegenerative diseases. Here, we aim to expand current binding models to help explain the specificity seen among three activators of α4β2 receptors: sazetidine-A, cytisine, and NS9283. Through mutational analysis, we can interchange the activation profiles of the stoichiometry-selective compounds sazetidine-A and cytisine. In addition, mutations render NS9283—currently identified as a positive allosteric modulator—into an agonist. These results lead to two conclusions: (1) occupation at each primary face of an α subunit is needed to activate the channel and (2) the complementary face of the adjacent subunit dictates the binding ability of the agonist.

An Accessory Agonist Binding Site Promotes Activation of α4β2* Nicotinic Acetylcholine Receptors

“…NS9283 can activate nAChRs into which three ␣4/␣4 sites have been engineered (16,17). However, NS9283 did not activate wild type nAChRs when delivered alone (12) (Fig.…”

“…NS9283 selectively binds to the accessory ␣4/␣4 agonist site, acting as a third agonist to increase the probability of channel opening (16,17) (Fig. 4).…”

“…However, it was recently established that NS9283 is neither allosteric nor a modulator. It is not allosteric because NS9283 acts as a selective agonist at the ACh binding site formed at the ␣4/␣4 interface (16,17). NS9283 cannot activate through its action on the ␣4/␣4 site alone.…”

“…The drug sazetidine is a full agonist at ␣4/␤2 primary agonist sites but does not bind to the ␣4/␣4 accessory ACh binding site (20 -22). Cytisine acts at both types of ACh binding sites as a partial agonist (16,20,22). Dihydro-␤-erythroidine is an antagonist at both ␣4/␤2 and ␣4/␣4 ACh binding sites (3).…”

The Nicotine Metabolite, Cotinine, Alters the Assembly and Trafficking of a Subset of Nicotinic Acetylcholine Receptors

“…These drugs include the partial agonists cytisine (19) and varenicline (20) as well as the antagonist mecamylamine (21). Although nicotine exposure increases the assembly of the high sensitivity receptor, (␣4) 2 (␤2) 3 (16,22), cytisine has been shown to alter the stoichiometry of ␣4␤2 with a preference for the low sensitivity receptor, (␣4) 3 (␤2) 2 , potentially due to the existence of an additional binding site for cytisine at the ␣-␣ interface (23,24). The high and low sensitivity stoichiometries of ␣4␤2 exhibit different ligand binding affinities and Ca 2ϩ flux and desensitize at different rates (5,9).…”