5-HT₃ receptors (version 2019.4) in the IUPHAR/BPS Guide to Pharmacology Database

Abstract

The 5-HT₃ receptor (nomenclature as agreed by the NC-IUPHAR Subcommittee on 5-Hydroxytryptamine (serotonin) receptors [66]) is a ligand-gated ion channel of the Cys-loop family that includes the zinc-activated channels, nicotinic acetylcholine, GABA_A and strychnine-sensitive glycine receptors. The receptor exists as a pentamer of 4TM subunits that form an intrinsic cation selective channel [5]. Five human 5-HT₃ receptor subunits have been cloned and homo-oligomeric assemblies of 5-HT₃A and hetero-oligomeric assemblies of 5-HT₃A and 5-HT₃B subunits have been characterised in detail. The 5-HT₃C (HTR3C, Q8WXA8), 5-HT₃D (HTR3D, Q70Z44) and 5-HT₃E (HTR3E, A5X5Y0) subunits [83, 122], like the 5-HT₃B subunit, do not form functional homomers, but are reported to assemble with the 5-HT₃A subunit to influence its functional expression rather than pharmacological profile [124, 63, 157]. 5-HT₃A, -C, -D, and -E subunits also interact with the chaperone RIC-3 which predominantly enhances the surface expression of homomeric 5-HT₃A receptor [157]. The co-expression of 5-HT₃A and 5-HT₃C-E subunits has been demonstrated in human colon [82]. A recombinant hetero-oligomeric 5-HT₃AB receptor has been reported to contain two copies of the 5-HT₃A subunit and three copies of the 5-HT₃B subunit in the order B-B-A-B-A [7], but this is inconsistent with recent reports which show at least one A-A interface [96, 150]. The 5-HT₃B subunit imparts distinctive biophysical properties upon hetero-oligomeric 5-HT₃AB versus homo-oligomeric 5-HT₃A recombinant receptors [32, 41, 56, 85, 139, 129, 79], influences the potency of channel blockers, but generally has only a modest effect upon the apparent affinity of agonists, or the affinity of antagonists ([17], but see [41, 30, 35]) which may be explained by the orthosteric binding site residing at an interface formed between 5-HT₃A subunits [96, 150]. However, 5-HT₃A and 5-HT₃AB receptors differ in their allosteric regulation by some general anaesthetic agents, small alcohols and indoles [138, 135, 71]. The potential diversity of 5-HT₃ receptors is increased by alternative splicing of the genes HTR3A and E [64, 19, 124, 123, 120]. In addition, the use of tissue-specific promoters driving expression from different transcriptional start sites has been reported for the HTR3A, HTR3B, HTR3D and HTR3E genes, which could result in 5-HT₃ subunits harbouring different N-termini [152, 79, 120]. To date, inclusion of the 5-HT₃A subunit appears imperative for 5-HT₃ receptor function.