@article {Lansdell87, author = {Stuart J. Lansdell and Chaitra Sathyaprakash and Anne Doward and Neil S. Millar}, title = {Activation of Human 5-Hydroxytryptamine Type 3 Receptors via an Allosteric Transmembrane Site}, volume = {87}, number = {1}, pages = {87--95}, year = {2015}, doi = {10.1124/mol.114.094540}, publisher = {American Society for Pharmacology and Experimental Therapeutics}, 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 {\textpm} 7\% and 80 {\textpm} 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.}, issn = {0026-895X}, URL = {https://molpharm.aspetjournals.org/content/87/1/87}, eprint = {https://molpharm.aspetjournals.org/content/87/1/87.full.pdf}, journal = {Molecular Pharmacology} }