The analgesic-like properties of the alpha7 nAChR silent agonist NS6740 is associated with non-conducting conformations of the receptor

Abstract

The alpha 7 nicotinic acetylcholine receptor (nAChR) is a promising drug target for a number of neurological disorders including chronic pain and inflammatory diseases. Since alpha 7 can function as a ligand-gated ion channel, drug development initially focused on ligands that were selective activators of the alpha 7 ion channel. However, the best alpha 7 drugs for chronic pain and inflammation indications may not be ion channel activators but rather "silent agonists", which bind to the receptor but preferentially induce non-conducting states that modulate signal transduction in non-neuronal cells. One such compound is NS6740. We show that NS6740 selectively induces prolonged desensitization of alpha 7 nAChRs. There are two forms of alpha 7 desensitization that can be distinguished by their sensitivity to the positive allosteric modulators (PAMs). At high concentrations, NS6740 preferentially induces PAM-insensitive desensitization, which over the course of several minutes reverts to the sensitive form. NS6740 was tested in several pain models after in vivo administration in the mouse. Although it had no effects in acute thermal pain, NS6740 induced significant dose- and time-dependent antinociceptive activity in formalin- and acetic acid-induced nociceptive behaviors as well as in the chronic constrictive nerve injury (CCI) model for neuropathic pain. The antinociceptive activity of NS6740 in these models was alpha 7-dependent. In addition, NS6740 administration reversed pain-induced aversion, an important affective component of pain. The time and concentration dependence of the effects were consistent with NS6740 induction of PAM-insensitive non-conducting states, suggesting that signal transduction required for analgesia is accomplished by alpha 7 receptors in that conformation.