Orthosteric and/or Allosteric Binding of -Conotoxins to Nicotinic Acetylcholine Receptors and Their Models

-Conotoxins from Conus snails are capable of distinguishing muscle and neuronal nicotinic acetylcholine receptors (nAChRs). -Conotoxin RgIA and O-conotoxin GeXIVA, blocking neuronal 910 nAChR, are potential analgesics. Typically, -conotoxins bind to the orthosteric sites for agonists/competitive antagonists, but O-conotoxin GeXIVA was proposed to attach allosterically, judging by electrophysiological experiments on 910 nAChR. We decided to verify this conclusion by radioligand analysis in competition with -bungarotoxin (Bgt) on the ligand-binding domain of the nAChR 9 subunit (9 LBD), where, from the X-ray analysis, Bgt binds at the orthosteric site. A competition with Bgt was registered for GeXIVA and RgIA, IC50 values being in the micromolar range. However, high nonspecific binding of conotoxins (detected with their radioiodinated derivatives) to His(6)-resin attaching 9 LBD did not allow us to accurately measure IC(50)s. However, IC(50)s were measured for binding to Aplysia californica AChBP: the RgIA globular isomer, known to be active against 910 nAChR, was more efficient than the ribbon one, whereas all three GeXIVA isomers had similar potencies at low mu M. Thus, radioligand analysis indicated that both conotoxins can attach to the orthosteric sites in these nAChR models, which should be taken into account in the design of analgesics on the basis of these conotoxins.