A Series of α7 Nicotinic Acetylcholine Receptor Allosteric Modulators with Close Chemical Similarity but Diverse Pharmacological Properties

Abstract

Acetylcholine activates nicotinic acetylcholine receptors (nAChRs) by binding to an extracellular site located at the interface of two adjacent subunits. In contrast, recent studies have provided evidence that positive allosteric modulators (PAMs) such as TQS (4-(naphthalen-2-yl)-3a,4,5,9b-tetrahydro-3H-cyclopenta[c]quinoline-8-sulfonamide) and allosteric agonists such as 4BP-TQS (4-(4-bromophenyl)-3a,4,5,9b-tetrahydro-3H-cyclopenta[c]quinoline-8-sulfonamide) interact at an intrasubunit transmembrane site. Here, we describe the synthesis and pharmacological characterization of a series of chemically related allosteric modulators of the ␣7 nAChR. Minimal changes in the chemical structure of these compounds have been found to exert profound effects on their pharmacological properties. For example, compounds containing a bromine atom at either the ortho or meta position on the phenyl ring, such as 2BP-TQS (4-(2-bromophenyl)-3a,4,5,9b-tetrahydro-3H-cyclopenta[c]quinoline-8-sulfonamide) and 3BP-TQS (4-(3-bromophenyl)-3a,4,5,9b-tetrahydro-3H-cyclopenta[c]quinoline-8-sulfonamide), rather than at the para position (4BP-TQS), display no allosteric agonist activity but retain PAM activity on ␣7 nAChRs, demonstrating the importance of the location of the halogen atom on pharmacological properties. Replacement of the bromine atom in 4BP-TQS with either a chlorine [4CP-TQS (4-(4-chloroophenyl)-3a,4,5,9b-tetrahydro-3H-cyclopenta[c]quinoline-8-sulfonamide)] or an iodine atom [4IP-TQS (4-(4-iodoophenyl)-3a,4,5,9b-tetrahydro-3H-cyclopenta[c]quinoline-8-sulfonamide)] results in compounds that have pharmacological properties characteristic of allosteric agonists but display differences in activation rates, in inactivation rates, and in levels of desensitization. In contrast, replacement of the bromine atom in 4BP-TQS with a fluorine atom [4FP-TQS (4-(4-fluorophenyl)-3a,4,5,9b-tetrahydro-3H-cyclopenta[c]quinoline-8-sulfonamide)] generated a compound that lacks allosteric agonist activity but acts a potentiator of responses to acetylcholine. In addition, 4FP-TQS was found to act as an antagonist of responses evoked by allosteric agonists such as 4BP-TQS. These findings provide evidence of the pharmacological diversity of compounds interacting with the allosteric transmembrane site on ␣7 nAChRs.

Activation of Human 5-Hydroxytryptamine Type 3 Receptors via an Allosteric Transmembrane Site

“…The onset of activation by these compounds is significantly slower than with activation by the orthosteric agonist 5-HT, a finding that has been reported with orthosteric and allosteric (transmembrane) agonists of nAChRs (Gill et al, 2011(Gill et al, , 2012. It is also consistent with studies obtained with chimeric and mutated 5-HT 3 R subunits.…”

“…Carvacrol and thymol activate human 5-HT 3 Rs with a significantly slower onset than the endogenous orthosteric agonist 5-HT (P , 0.001). A similar difference in the onset of receptor activation has been observed with nAChRs activated by either the orthosteric agonist acetylcholine or by allosteric agonists acting via transmembrane binding site (Gill et al, 2011(Gill et al, , 2012. showing all five subunits of the pentameric receptor, viewed from the extracellular side, in which the backbone a-helices of the two subunits that were used for computer docking studies are shaded in light blue and green.…”

“…However, there is increasing evidence indicating that Cys-loop receptors can be activated by allosteric ligands, interacting at sites that are distinct from the orthosteric site. For example, nAChRs can be activated by ligands binding to a transmembrane allosteric site in the absence of orthosteric agonists such as acetylcholine (Gill et al, 2011(Gill et al, , 2012. In addition, ivermectin, a macrocyclic lactone, activates glutamate-gated chloride channels in the absence of the endogenous agonist glutamate (Cully et al, 1994(Cully et al, , 1996 via a transmembrane binding site (Hibbs and Gouaux, 2011).…”

“…For example, a large number of positive and negative allosteric modulators of nAChRs have been identified and there is evidence that at least some of these interact with an intrasubunit transmembrane site (Young et al, 2008;Collins et al, 2011). More recently, a group of nAChR allosteric agonists have been identified that cause receptor activation in the absence of an orthosteric agonist and have been proposed to act via an intrasubunit transmembrane site (Gill et al, 2011(Gill et al, , 2012(Gill et al, , 2013. Similarly, there is evidence for allosteric activation of other Cys-loop receptors including GABA A Rs (Amin and Weiss, 1993) and glutamate-gated chloride channels (Cully et al, 1994(Cully et al, , 1996.…”

Lethal effects of an insecticidal spider venom peptide involve positive allosteric modulation of insect nicotinic acetylcholine receptors

“…This secondary slower component has also been observed with other nAChR type II PAMs, where it has been attributed to the ability of the transmitter to reach the orthosteric site faster than the PAM reaches the allosteric site (Gill et al, 2012). However, this explanation seems unlikely since the currents were recorded in the sustained presence of toxin with intermittent exposure to nicotine.…”