Williams_2012_Mol.Pharmacol_82_746

alpha7 nicotinic acetylcholine receptors (nAChRs) have been a puzzle since their discovery in brain and non-neuronal tissues. Maximal transient probability of an alpha7 nAChR being open with rapid agonist applications is only 0.002. The concentration dependence of alpha7 responses measured from transfected cells and Xenopus laevis oocytes shows the same disparity in potency estimations for peak currents and net charge, despite being studied at 1000-fold different time scales. In both cases the EC(5)(0) was approximately 10-fold lower for net charge than for peak currents. The equivalence of the data obtained at such disparate time scales indicates that desensitization of alpha7 is nearly instantaneous. At high levels of agonist occupancy, the receptor is preferentially converted to a ligand-bound nonconducting state, which can be destabilized by the positive allosteric modulator N-(5-chloro-2,4-dimethoxyphenyl)-N'-(5-methyl-3-isoxazolyl)-urea (PNU-120596). Such currents can be sufficiently large to be cytotoxic to the alpha7-expressing cells. Both the potentiating effect of PNU-120596 and the associated cytotoxicity have a high temperature dependence that can be compensated for by serum factors. Therefore, despite reduced potentiation at body temperatures, use of type II positive allosteric modulators may put cells that naturally express high levels of alpha7 nAChRs, such as neurons in the hippocampus and hypothalamus, at risk. With a low intrinsic open probability and high propensity toward the induction of nonconducting ligand-bound states, it is likely that the well documented regulation of signal transduction pathways by alpha7 nAChRs in cells such as those that regulate inflammation may be independent of ion channel activation and associated with the nonconducting conformational states.