title: Activation of human 5-hydroxytryptamine type 3 receptors via an allosteric transmembrane site.
abstract: In common with other members of the Cys-loop family of pentameric ligand-gated ion channels, 5-hydroxytryptamine type 3 receptors (5-HT3Rs) are activated by the binding of a neurotransmitter to an extracellular orthosteric site, located at the interface of two adjacent receptor subunits. In addition, a variety of compounds have been identified that modulate agonist-evoked responses of 5-HT3Rs, and other Cys-loop receptors, by binding to distinct allosteric sites. In this study, we examined the pharmacological effects of a group of monoterpene compounds on recombinant 5-HT3Rs expressed in Xenopus oocytes. Two phenolic monoterpenes (carvacrol and thymol) display allosteric agonist activity on human homomeric 5-HT3ARs (64 ± 7% and 80 ± 4% of the maximum response evoked by the endogenous orthosteric agonist 5-HT, respectively). In addition, at lower concentrations, where agonist effects are less apparent, carvacrol and thymol act as potentiators of responses evoked by submaximal concentrations of 5-HT. By contrast, carvacrol and thymol have no agonist or potentiating activity on the closely related mouse 5-HT3ARs. Using subunit chimeras containing regions of the human and mouse 5-HT3A subunits, and by use of site-directed mutagenesis, we have identified transmembrane amino acids that either abolish the agonist activity of carvacrol and thymol on human 5-HT3ARs or are able to confer this property on mouse 5-HT3ARs. By contrast, these mutations have no significant effect on orthosteric activation of 5-HT3ARs by 5-HT. We conclude that 5-HT3ARs can be activated by the binding of ligands to an allosteric transmembrane site, a conclusion that is supported by computer docking studies.
authors.affiliation: Department of Neuroscience, Physiology, and Pharmacology, University College London, London, United Kingdom.
authors.forename: Stuart J
authors.initials: SJ
authors.lastname: Lansdell
authors.affiliation: Department of Neuroscience, Physiology, and Pharmacology, University College London, London, United Kingdom.
authors.forename: Chaitra
authors.initials: C
authors.lastname: Sathyaprakash
authors.affiliation: Department of Neuroscience, Physiology, and Pharmacology, University College London, London, United Kingdom.
authors.forename: Anne
authors.initials: A
authors.lastname: Doward
authors.affiliation: Department of Neuroscience, Physiology, and Pharmacology, University College London, London, United Kingdom.
authors.forename: Neil S
authors.identifier: http://orcid.org/0000-0003-3318-4152
authors.initials: NS
authors.lastname: Millar
mesh_terms: D000596:Amino Acids; D000818:Animals; D001665:Binding Sites; D000081007:Cymenes; D004357:Drug Synergism; D057809:HEK293 Cells; D006801:Humans; D051379:Mice; D062105:Molecular Docking Simulation; D039821:Monoterpenes / Q000494:pharmacology*; D016297:Mutagenesis, Site-Directed; D044406:Receptors, Serotonin, 5-HT3 / Q000235:genetics / Q000378:metabolism*; D058827:Serotonin 5-HT3 Receptor Agonists / Q000494:pharmacology*; D013045:Species Specificity; D013943:Thymol / Q000494:pharmacology; D014982:Xenopus laevis / Q000196:embryology* / Q000235:genetics
chemical_list: D000596:Amino Acids; D000081007:Cymenes; D039821:Monoterpenes; D044406:Receptors, Serotonin, 5-HT3; D058827:Serotonin 5-HT3 Receptor Agonists; D013943:Thymol; C073316:carvacrol