Allosteric modulation of Torpedo nicotinic acetylcholine receptor ion channel activity by noncompetitive agonists

Similar to other neuroreceptors of the vertebrate central nervous system, the nicotinic acetylcholine receptor (nAChR) is subject to modulatory control by allosterically acting ligands. Of particular interest in this regard are allosteric ligands that enhance the sensitivity of the receptor to its natural agonist acetylcholine (ACh), as such ligands could be useful as drugs in diseases associated with impaired nicotinic neurotransmission. Here we discuss the action of a novel class of nAChR ligands which act as allosterically potentiating ligands (APL) on the nicotinic responses induced by ACh and competitive agonists. In addition, APLs also act as noncompetitive agonists of very low efficacy, and as direct blockers of ACh-activated channels. These actions are observed with nAChRs from brain, muscle and electric tissue, and they depend on the structure of the APL and the concentration range applied. We focus here on Torpedo nAChR because (i) the unusual pharmacology of these ligands was first discovered with this system, and (ii) large quantities of this receptor are readily available for biochemical studies.