Received for publication October 19, 2007.
Revised July 8, 2008.
Accepted for publication August 1, 2008.

Positive Allosteric Modulation of the 7 nAChR: Ligand Interactions with Distinct Binding Sites and Evidence for a Prominent Role of the M2-M3 Segment 7 nAChR: Ligand Interactions with Distinct Binding Sites and Evidence for a Prominent Role of the M2-M3 Segment

Abstract

The 7 nicotinic acetylcholine receptor (nAChR), a homopentameric, rapidly activating and desensitizing ligand-gated ion channel with relatively high degree of calcium permeability, is expressed in the mammalian central nervous system including in regions associated with cognitive processing. Selective agonists targeted the 7 nAChR have shown efficacy in animal models of cognitive dysfunction. Use of positive allosteric modulators selective for the 7 receptor is another strategy that is envisaged in the design of active compounds aiming at improving cognitive and sensory gating dysfunction. The recent discovery of novel positive allosteric modulators such as the NS-1738 and PNU-120596 that are selective for the 7 nAChRs but display significant phenotypic differences in their profile of allosteric modulation suggests that these molecules may act at different sites on the receptor. Taking advantage of the possibility to obtain functional receptors by the fusion of proteins domains from the 7 and the 5-HT³ receptor, we examined the structural determinants required for positive allosteric modulation. This strategy revealed that the extracellular N-terminal domain of 7 plays a critical role in allosteric modulation by NS-1738. In addition, 7-5HT³ chimeras harboring the M2-M3 segment showed that spontaneous activity in response to NS-1738, which confirmed the critical contribution of this small extracellular segment in the receptor gating. In contrast to NS-1738, positive allosteric modulation by PNU-120596 could not be restored in the 7-5HT³ chimeras, but was selectively observed in the reverse 5HT³- 7 chimera. Altogether, these data illustrate the existence of distinct allosteric binding sites with specificity of different profiles of allosteric modulators, and open new possibilities to investigate the 7 receptor function.

Key words: Nicotinic cholinergic, Ion channel regulation, Synaptic plasticity