Catharanthine Alkaloids are Noncompetitive Inhibitors of Muscle-Type Nicotinic Acetylcholine Receptors

Abstract

This is an attempt to characterize the binding sites and the mechanisms of action of several catharanthine alkaloids including ibogaine, vincristine, and vinblastine, on muscle-type nicotinic acetylcholine receptors (AChRs), by comparing their pharmacological properties with that for the well characterized high-affinity noncompetitive antagonist (NCA) phencyclidine (PCP). In this regard, structural and functional approaches were used including radioligand equilibrium and competition binding assays using [3H]ibogaine and the analog of PCP, [piperidyl-3, 4-3H(N)]-N-(1-(2 thienyl)cyclohexyl)-3,4-piperidine ([3H]TCP), Ca2+ influx determinations, thermodynamic and kinetic measurements using column-immobilized Torpedo AChRs, and molecular docking and dynamics studies The results established that: (a) the alkaloidsinhibit (±)-epibatidine-induced Ca2+ influx in embryonic muscle AChRs with the following potencies (in μM): ibogaine (17 ± 3) > vinblastine (20 ± 5) > vincristine (25 ± 4), that are slightly higher than that for PCP (31 ± 2), (b) the alkaloids inhibit [3H]TCP binding, and ibogaine and PCP inhibit [3H]ibogaine binding, to the desensitized Torpedo AChR with higher affinity compared to the resting AChR, (c) ibogaine binds to the Torpedo AChR by an entropy-driven process, and (d) ibogaine interacts with a binding domain located between the serine (position 6’) and valine (position 13’) rings, by a network of van der Waals and polar interactions. Collectively our data indicate that catharanthine alkaloids block agonist-activated ion channels by interacting with a binding domain that is shared with PCP located between the serine and valine rings. This supports the view that the catharanthine moiety, which is shared by ibogaine and vinca alkaloids, is a minimum structural requirement for the interaction of these molecules with the ion channel. In addition, ibogaine and vinca alkaloids may induce and maintain the desensitized state by a mechanism where their dissociation rates are decreased.