Catharanthine alkaloids are noncompetitive antagonists of muscle-type nicotinic acetylcholine receptors.

Catharanthine alkaloids such as ibogaine, vincristine, and vinblastine inhibit muscle nicotinic acetylcholine receptors (AChRs) in a noncompetitive manner, blocking ion flow and promoting receptor desensitization. These compounds inhibit epibatidine-induced calcium influx in TE671 cells with similar potencies (IC50 = 17–25 μM). They bind more tightly to desensitized than resting AChRs and enhance binding of cytisine to resting receptors, indicating desensitizing properties. Phencyclidine (PCP) inhibits ibogaine binding to the AChR through steric hindrance. Docking experiments suggest neutral ibogaine forms hydrogen bonds with the serine ring at position 6', a site shared with PCP, while protonated ibogaine may form a salt bridge with acidic residues at the outer ring. The catharanthine moiety is the minimal structure required for AChR inhibition.

Study at a glance

Abstract

We compared the interaction of several catharanthine alkaloids including, ibogaine, vincristine, and vinblastine, with that for the noncompetitive antagonist phencyclidine (PCP) at muscle nicotinic acetylcholine receptors (AChRs) in different conformational states. The results established that catharanthine alkaloids: (a) inhibit, in a noncompetitive manner, (+/-)-epibatidine-induced Ca(2+) influx in TE671-halpha1beta1gammadelta cells with similar potencies (IC(50)=17-25microM), (b) inhibit [(3)H]TCP binding to the desensitized Torpedo AChR with higher affinity compared to the resting AChR, and (c) enhance [(3)H]cytisine binding to resting but activatable Torpedo AChRs, suggesting desensitizing properties. Interestingly, PCP inhibits [(3)H]ibogaine binding to the AChR in a steric fashion. This is corroborated by additional docking experiments indicating that the amino groups of neutral ibogaine form hydrogen bonds with the serine ring (position 6'), a location shared with PCP. Since protonated ibogaine forms a salt bridge with one of the acidic residues at the outer ring (position 20'), this ligand could be first attracted to the entrance of the channel by electrostatic interactions. Our data indicate that the catharanthine moiety is a minimum structural requirement for AChR inhibition including, ion channel blocking and desensitization, and that ibogaine and PCP bind to overlapping sites in the desensitized AChR ion channel.