Molecular determinants of pyrantel selectivity in nicotinic receptors.
Nicotinic receptors (acetylcholine receptors, AChRs) play key roles in synaptic transmission throughout the nervous system. AChRs mediate neuromuscular transmission in nematodes, and they are targets for antiparasitic drugs. The anthelmintic agents levamisole and pyrantel, which are potent agonists of nematode muscle AChRs, are partial agonists of mammalian muscle AChRs. To further explore the structural basis of the differential activation of AChR subtypes by anthelmintics, we studied the activation of alpha7 AChRs using the high-conductance form of the alpha7-5-hydroxytryptamine-3A receptor, which is a good model for pharmacological studies involving the extracellular region of alpha7. Macroscopic and single-channel current recordings show that levamisole is a weak agonist of alpha7. It is interesting that pyrantel is a more potent agonist of alpha7 than acetylcholine (ACh). To identify determinants of this differential activation, we replaced residues of the complementary face of the binding site by the homologous residues in the muscle epsilon subunit and evaluated changes in activation. The mutation Q57G does not affect the activation by either ACh or levamisole. However, it increases EC50 values and decreases the maximal response to pyrantel. Kinetic analysis shows that gating of the mutant channel activated by pyrantel is profoundly impaired. The decreased sensitivity of alpha7-Q57G to pyrantel agrees with its weak action at muscle AChRs, indicating that when glycine occupies position 57, as in the mammalian muscle AChR, pyrantel behaves as a partial agonist. Thus, position 57 located at the complementary face of the binding site plays a key role in the selective activation of AChRs by pyrantel.
Bartos
Mariana
M
Instituto de Investigaciones Bioquímicas, Universidad Nacional del Sur-Consejo de Investigaciones Científicas y Técnicas, Camino La Carrindanga Km 7, B8000FWB, Bahía Blanca, Argentina.
Rayes
Diego
D
Bouzat
Cecilia
C
D000109:Acetylcholine / Q000494:pharmacology; D000595:Amino Acid Sequence; D000818:Animals; D000871:Anthelmintics / Q000494:pharmacology; D001665:Binding Sites; D006801:Humans; D007978:Levamisole / Q000494:pharmacology*; D008564:Membrane Potentials; D008954:Models, Biological; D008969:Molecular Sequence Data; D018722:Nicotinic Agonists / Q000494:pharmacology*; D011715:Pyrantel / Q000494:pharmacology*; D011978:Receptors, Nicotinic / Q000737:chemistry*; D011985:Receptors, Serotonin / Q000235:genetics / Q000378:metabolism*; D044406:Receptors, Serotonin, 5-HT3; D017386:Sequence Homology, Amino Acid
D000871:Anthelmintics; C509409:HTR3A protein, human; D018722:Nicotinic Agonists; D011978:Receptors, Nicotinic; D011985:Receptors, Serotonin; D044406:Receptors, Serotonin, 5-HT3; D007978:Levamisole; D011715:Pyrantel; D000109:Acetylcholine