Pharmacological characteristics and binding modes of caracurine V analogues and related compounds at the neuronal α7 nicotinic acetylcholine receptor

The pharmacological properties of bisquaternary caracurine V, iso-caracurine V, and pyrazino[l,2-a;4,5-a ']diindole analogues and of the neuromuscular blocking agents alcuronium and toxiferine I have been characterized at numerous ligand-gated ion channels. Several of the analogues are potent antagonists of the homomeric alpha 7 nicotinic acetylcholine receptor (nAChr), displaying nanomolar binding affinities and inhibiting acetylcholine-evoked signaling through the receptor in a competitive manner. In contrast, they do not display activities at heteromeric neuronal nAChRs and only exhibit weak antagonistic activities at the related 5-HT3A serotonin receptor. In a mutagenesis study, five selected analogues have been demonstrated to bind to the orthosteric site of the 0 nAChR. The binding site of the compounds overlaps with that of the standard 0 antagonist methyllycaconitine, the binding of them being centered in a cation-pi interaction between the quaternary nitrogen atom of the ligand and the Trp(149) residue in the receptor, with additional key contributions from other aromatic receptor residues such as Tyr(188), Tyr(195), and Trp(55).