Glutamine 57 at the Complementary Binding Site Face Is a Key Determinant of Morantel Selectivity for α7 Nicotinic Receptors

Abstract

Nicotinic receptors (AChRs) play key roles in synaptic transmission. We explored activation of neuronal alpha7 and mammalian muscle AChRs by morantel and oxantel. Our results revealed a novel action of morantel as a high-efficacy and more potent agonist than ACh of alpha7 receptors. The EC50 for activation by morantel of both alpha7 and alpha7-5HT3A receptors is 7-fold lower than that determined for ACh. The minimum morantel concentration required to activate alpha7-5HT3A channels is 6-fold lower than that of ACh, and activation episodes are more prolonged than in the presence of ACh. By contrast, oxantel is a weak agonist of alpha7 and alpha7-5HT3A, and both drugs are very low-efficacy agonists of muscle AChRs. The replacement of Gln57 in alpha7 by glycine, which is found in the equivalent position of the muscle AChR, decreases the efficacy for activation and turns morantel into a partial agonist. The reverse mutation in the muscle AChR (epsilonG57Q) increases 7-fold the efficacy of morantel. The mutations do not affect activation by ACh or oxantel, indicating that this position is selective for morantel. In silico studies show that the tetrahydropyrimidinyl group, common to both drugs, is close to W149 of the principal face of the binding site, whereas the other cyclic group is proximal to Q57 of the complementary face in morantel but not in oxantel. Thus, position 57 at the complementary face is a key determinant of the high selectivity of morantel for alpha7. These results provide new information for further progress in drug design.