Noncompetitive inhibition of nicotinic acetylcholine receptors by endogenous molecules

Over the past few decades much effort has been expended elucidating the key domains of the nicotinic acetylcholine receptor (AChR) responsible for agonist binding, ion conduction, and gating. An emerging concept in the receptor field has been to consider the receptor entity as a signal transducer that suffers modulatory control by allosterically acting ligands. Of particular interest are the molecules that inhibit the agonist-evoked ion flux activity in a noncompetitive manner: the so-called noncompetitive inhibitors (NCIs). The actual knowledge on the action of NCIs was obtained by using several drugs from exogenous origin. However, several lines of investigation indicate that the receptor protein can be modulated by endogenous substances other than acetylcholine. In this regard, we outline the progress evidenced on the localization of binding sites for drugs of endogenous origin that have been found to directly interact with the AChR in a noncompetitive fashion. Among them we can quote lipids such as steroids and fatty acids, the neurotransmitter 5-hydroxytryptamine (5-HT) and related compounds, as well as the neuropeptide substance P. We present the available experimental evidence indicating the existence of both luminal (located into the ion channel) and nonluminal (located out of the ion channel) binding sites for endogenous NCIs. Particularly, the binding site for substance P is found in the delta M2 domain. In addition, the locus for 5-HT is putatively located in the ion channel close to the serine ring, whereas the binding site for two competitive antagonists of 5-HT receptors (e.g., methysergide and spiperone) is located closer to the external end of the ion channel. Instead, fatty acid and steroid molecules bind to nonluminal sites. More specifically, fatty acids may bind to the annular lipid domain of the AChR or/and to the high-affinity quinacrine site (a NCI from exogenous origin) which is located at a nonannular lipid domain. Additionally, steroids may bind to a site located on the extracellular hydrophilic domain of the AChR or/and at the lipid-protein interface, specifically, at the annular lipid domain and/or close to the nonannular quinacrine binding site.