Coronaridine congeners inhibit human α3β4 nicotinic acetylcholine receptors by interacting with luminal and non-luminal sites

To characterize the interaction of coronaridine congeners with human (h) α3β4 nicotinic acetylcholine receptors (AChRs), structural and functional approaches were used. The Ca²⁺ influx results established that coronaridine congeners noncompetitively inhibit hα3β4 AChRs with the following potency (IC₅₀'s in μM) sequence: (-)-ibogamine (0.62 ± 0.23) ∼ (+)-catharanthine (0.68 ± 0.10) > (-)-ibogaine (0.95 ± 0.10) > (±)-18-methoxycoronaridine [(±)-18-MC] (1.47 ± 0.21) > (-)-voacangine (2.28 ± 0.33) > (±)-18-methylaminocoronaridine (2.62 ± 0.57 μM) ∼ (±)-18-hydroxycoronaridine (2.81 ± 0.54) > (-)-noribogaine (6.82 ± 0.78). A good linear correlation (r² = 0.771) between the calculated IC₅₀ values and their polar surface area was found, suggesting that this is an important structural feature for its activity. The radioligand competition results indicate that (±)-18-MC and (-)-ibogaine partially inhibit [³H]imipramine binding by an allosteric mechanism. Molecular docking, molecular dynamics, and in silico mutation results suggest that protonated (-)-18-MC binds to luminal [i.e., β4-Phe255 (phenylalanine/valine ring; position 13′), and α3-Leu250 and β4-Leu251 (leucine ring; position 9′)], non-luminal, and intersubunit sites. The pharmacophore model suggests that nitrogens from the ibogamine core as well as methylamino, hydroxyl, and methoxyl moieties at position 18 form hydrogen bonds. Collectively our data indicate that coronaridine congeners inhibit hα3β4 AChRs by blocking the ion channel's lumen and probably by additional negative allosteric mechanisms by interacting with a series of non-luminal sites.