α-7 Nicotinic Receptor Positive Allosteric Modulators have Varying Kinetic Effects on Desensitization and Current Amplitude

Abstract

The α-7 nicotinic receptor is an important neuronal subtype of ligand gated ion channels. It forms a pentameric homomer that is activated by acetylcholine or nicotine to evoke rapidly activating and desensitizing currents. Activation of α-7 receptors has been implicated as a therapeutic strategy in schizophrenia and Alzheimer disease. Small molecule positive allosteric modulators (PAM's) have been shown to enhance α-7 currents, and are classified as type 1 or type 2 PAMs. The type 1 PAMs enhance the current amplitude but do not alter desensitization, while the type 2 PAMs enhance the current amplitude but also slow the desensitization of the receptor. In order to understand the mechanism of action of the type 1 and type 2 PAM's, we tested three modulators on the QPatch automated patch-clamp system using GH4C1 cells stably expressing the rat α-7 receptor. We also explored the effect of repeated applications of these agents on their modulatory activity. Consistent with previously published data, PNU-120596 showed type 2 PAM activity accompanied with a decreasing magnitude of potentiation with repeated applications. Estimated EC50 values for PNU-120596 were stable with repeated compound application. NS-1738 produced a type 1 PAM activity, and a cumulatively increased potentiation following repeated applications, accompanied by an approximate 3-fold increase in EC50. In contrast, SB-206553 had similar potency and effects on the potentiation with repeated application. These results indicate that the different α-7 receptor PAMs have different rates of activation and desensitization, in addition to their type 1 or 2 effects on receptor desensitization.