Allosteric activation of the human 5-HT3 receptor

Abstract

The serotonin type 3 receptors (5-HT3 ) are cation-selective chan- nels that belong to the Cys-loop receptor family. They are involved in fast excitatory transmission in central and peripheral nervous sys- tems and are implicated in gastrointestinal and neurological func- tions. Five different subunits (A-E) have been identified in humans, and the A subunit is the only one capable of forming functional ho- mopentameric receptors (5-HT3 A). These receptors are activated by agonist binding to the orthosteric sites located at the interfaces between two adjacent subunits at the extracellular region. Ago-pos- itive allosteric modulators (Ago-PAMs) are ligands that bind to an allosteric binding site and mediate a receptor response in the ab- sence of an orthosteric agonist and also potentiate its response. We here used the high-conductance form of the receptor (5-HT3 AHC), which allows detection of single-channel openings from patch-clamp recordings, to determine the molecular basis underlying its activa- tion and modulation by two ago-PAMs, thymol and carvacrol. From cell-attached recordings, we observed that both ligands activate the receptor in a similar way, eliciting openings in quick succession that are grouped in episodes (bursts) of high open probability (>0.9). The combined application of orthosteric agonists (full or partial) with the allosteric ligands elicits single channel events whose mean open and burst durations are intermediate between those obtained when orthosteric or allosteric agonists are applied individually. Our results reveal the mechanistic basis underlying activation and modulation of the 5-HT3 A receptor by two ago-positive allosteric modulators, thus providing new information that is essential