Activation of alpha-7 nicotinic receptors by anthelmintic agents

M. BARTOS; D. RAYES; CECILIA BEATRIZ BOUZAT

Salt Lake city

Congreso; 50th Annual Meeting Biophysical Society; 2006

Nicotinic 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. We have shown that the anthelminitic agents levamisole and pyrantel, which are potent agonists of nematode muscle AChRs, are low efficacious agonists of mammalian muscle AChRs (Rayes et al., 2004; J. Biol. Chem. 279). To understand the structural basis of the differential activation of anthelmintics among AChR subtypes, we here studied the activation of alfa7 AChRs by these agents at the single-channel and macroscopic-current levels. We used the high conductance form of the chimeric alfa7-5HT3A receptor (Rayes et al., 2005, Mol. Pharmacol., In press), which is a good model for pharmacological and single-channel studies involving the extracellular region of alfa7. Macroscopic and single-channel current recordings show that levamisole acts as a weak agonist of alfa7, whereas pyrantel is more potent than ACh (EC50ACh= 200 µM, EC50pyr= 45 µM). To identify determinants of this differential activation, we mutated residues of the complementary face of the binding site by the equivalent in the muscle alfa subunit and evaluated changes in activation. The mutation Q57G does not affect significantly the activation by ACh. However, it significantly increases the EC50 and decreases the maximal response to pyrantel. Single channels can be detected at 10-fold higher concentrations and the duration of bursts of openings significantly decreases with respect to the control chimera. The decreased sensitivity of Q57G-alfa7 to pyrantel agrees with its weak action at muscle AChRs, indicating that this residue may be involved in the differential activation of AChR subtypes by pyrantel.