The 5-HT3 receptor is a neurotransmitter-gated ion channel. It is a member of the Cys-loop family of receptors, which also includes nicotinic acetylcholine, glycine and GABAA receptors. Each member...

The 5-HT3 receptor is a member of the Cys-loop family of ligand-gated ion channels. These receptors are located in both the peripheral and central nervous systems, where functional receptors are constructed from five subunits. These subunits may be the same (homopentameric 5-HT3A receptors) or different (heteropentameric receptors, usually comprising of 5-HT3A and 5-HT3B receptor subunits), with the latter having a number of distinct properties. The 5-HT3 receptor binding site is comprised of six loops from two adjacent subunits, and critical ligand binding amino acids in these loops have been largely identified. There are a range of selective agonists and antagonists for these receptors and the pharmacophore is reasonably well understood. There are also a wide range of compounds that can modulate receptor activity. Studies have suggested many diverse potential disease targets that might be amenable to alleviation by 5-HT3 receptor selective compounds but to date only two applications have been fully realised in the clinic: the treatment of emesis and irritable-bowel syndrome.

5-Hydroxytryptamine type 3 (5-HT₃) receptors are cation-selective Cys loop receptors found in both the central and peripheral nervous systems. There are five 5-HT₃ receptor subunits (A–E), and all functional receptors require at least one A subunit. Regions from noncontiguous parts of the subunit sequence contribute to the agonist-binding site, and the roles of a range of amino acid residues that form the binding pocket have been identified. Drugs that selectively antagonize 5-HT₃ receptors (the “setrons”) are the current gold standard for treatment of chemotherapy-induced and postoperative nausea and vomiting and have potential for the treatment of a range of other conditions.

The binding pockets of Cys-loop receptors are dominated by aromatic amino acids. In the GABA_Areceptor α₁Phe65, β₂Tyr97, β₂Tyr157, and β₂Tyr205 are present at the β₂/α₁interface and have been implicated in forming an important part of the GABA binding site. Here, we have probed interactions of these residues using subtle chemical changes: unnatural amino acid mutagenesis was used to introduce a range of Phe analogs, and mutant receptors expressed in oocytes were studied using voltage-clamp electrophysiology. Serial mutations at β₂97 revealed a ∼20-fold increase in EC₅₀with the addition of each fluorine atom to a phenylalanine, indicating a cation–π interaction between GABA and this residue. This is the first example of a cation–π interaction in loop A of a Cys-loop receptor. Along with previous studies that identified cation–π interactions in loop B and loop C, the result emphasizes that the location of this interaction is not conserved in the Cys-loop family. The data further show that α₁65 (in loop D) is tolerant to subtle changes. Conversely, mutating either β₂Tyr157 (in loop B) or β₂Tyr205 (in loop C) to Phe substantially disrupts receptor function. Substitution of 4-F-Phe, however, at either position, or 4-MeO-Phe at β₂Tyr157, resulted in receptors with wild-type EC₅₀values, suggesting a possible hydrogen bond. The molecular scale insights provided by these data allow the construction of a model for GABA docking to the agonist binding site of the GABA_Areceptor.