Two Distinct Allosteric Binding Sites at α4β2 Nicotinic Acetylcholine Receptors Revealed by NS206 and NS9283 Give Unique Insights to Binding Activity-associated Linkage at Cys-loop Receptors

Background: Positive allosteric modulators (PAMs) of 42 nicotinic acetylcholine receptors have significant therapeutic potential. Results: Two PAMs, NS206 and NS9283, were observed to have differential and additive pharmacological actions due to binding at distinct receptor sites. Conclusion: Modulator binding activity is linked to the specific binding position in Cys-loop receptors. Significance: Diverse PAM profiles increase possibilities for rational drug design and understanding of receptor function. Positive allosteric modulators (PAMs) of 42 nicotinic acetylcholine receptors have the potential to improve cognitive function and alleviate pain. However, only a few selective PAMs of 42 receptors have been described limiting both pharmacological understanding and drug-discovery efforts. Here, we describe a novel selective PAM of 42 receptors, NS206, and compare with a previously reported PAM, NS9283. Using two-electrode voltage-clamp electrophysiology in Xenopus laevis oocytes, NS206 was observed to positively modulate acetylcholine (ACh)-evoked currents at both known 42 stoichiometries (2:3 and 3:2). In the presence of NS206, peak current amplitudes surpassed those of maximal efficacious ACh stimulations (E-max(ACh)) with no or limited effects at potencies and current waveforms (as inspected visually). This pharmacological action contrasted with that of NS9283, which only modulated the 3:2 receptor and acted by left shifting the ACh concentration-response relationship. Interestingly, the two modulators can act simultaneously in an additive manner at 3:2 receptors, which results in current levels exceeding E-max(ACh) and a left-shifted ACh concentration-response relationship. Through use of chimeric and point-mutated receptors, the binding site of NS206 was linked to the 4-subunit transmembrane domain, whereas binding of NS9283 was shown to be associated with the -interface in 3:2 receptors. Collectively, these data demonstrate the existence of two distinct modulatory sites in 42 receptors with unique pharmacological attributes that can act additively. Several allosteric sites have been identified within the family of Cys-loop receptors and with the present data, a detailed picture of allosteric modulatory mechanisms of these important receptors is emerging.