The 5-HT₃ receptor belongs to the Cys-loop superfamily of ligand-gated ion channels (LGICs). It consists of five subunits arranged around a central ion conducting pore, which is permeable to sodium (Na), potassium (K), and calcium (Ca) ions. 5-HT₃ receptor contains five subunits (HTR3A, HTR3B, HTR3C, HTR3D, HTR3E) and the genes are located on chromosomes 11 and 3 ^[1].

The 5-HT₃ receptor consists of five subunits arranged around a central ion conducting pore, which is permeable to sodium, potassium, and calcium ions. Binding of the neurotransmitter 5-hydroxytryptamine (serotonin) to the 5-HT₃ receptor opens the channel, which, in turn, leads to an excitatory response in neurons ^[2]. Each subunit comprises an extracellular N-terminal domain which comprises the orthosteric ligand-binding site. A transmembrane domain consisting of four interconnected alpha helices (M1-M4), with the extracellular M2-M3 loop involved in the gating mechanism. A large cytoplasmic domain between M3 and M4 involved in receptor trafficking and regulation; and a short extracellular C-terminus ^[2].

Yellow: residues E129 and F130 modify both binding and function, and changing F130 to asparagine even allows the receptor to be activated by acetylcholine, indicating this may be the most critical loop A residue ^[3].

The granisetron in the binding pocket, reveals the aromatic rings of granisetron lie between W183 and Y234 and the azabicyclic ring between W90 and F226 ^[4].

Anaesthetics may manifest some of their effects through 5-HT₃ receptors. Halothane and isoflurane, for example, potentiate 5-HT₃ receptor-mediated responses, and there is also evidence of modulation by anaesthetics as diverse as etomidate, ketamine and methohexital ^[5]. Studies using the chimaeric α7-nACh-5-HT₃ receptor suggest that halothane and isoflurane, and therefore possibly other anaesthetics, act at the extracellular N-terminal domain ^[6]. Local anaesthetics may also act at 5-HT₃ receptors: procaine, tetracaine, bupivacaine, lidocaine, cocaine and QX222, have been reported to inhibit 5-HT₃ receptor function ^[7].

Red: The most important B loop residue is W183, which is critical for both ligand binding and function. It has been extensively investigated using both natural and unnatural amino acid mutagenesis and these studies have revealed that W183 forms a cation-π bond with the primary amine of 5-HT ^[8][9].

Black: Aromatic residues are required at positions W90 and W95. The former is critical for ligand binding, whilst the latter affects cell surface expression ^[8][9].

Pink: Scanning alanine mutagenesis of loop E has revealed that Y143, G148, E149, V150, Q151, N152, Y153 and K154 may be important for granisetron binding, and indeed mutation of G148 and V150 completely abolished binding ^[10]. The two tyrosine residues Y143 and Y153 have been further studied using unnatural amino acid mutagenesis which has shown that they both play roles in function, and Y153 also has a role in ligand binding ^[11][12].

White: Mutational information is from studies which implicated W195 and S206 as potentially important residues ^[8][9].

Blue: The most important aromatic residue is probably Y234. It is involved in ligand binding whilst mutation of the closely positioned Y240 does not have any significant effects ^[11].

Reeves et al. found that 5-HT was orientated in the binding site with the charged primary amine located between W183 and Y234 at the top of the binding site and the hetero-aromatic rings between W90 and F226. This orientation is supported by mutagenesis data, and in particular by the evidence that W183 interacts with the primary amine via a cation-π interaction ^[13].

A wide range of substances, including divalent cations, alcohols, steroids and anaesthetics have been reported to modulate 5-HT₃ receptors, although their mechanisms of action are largely unknown. Modulation by divalent cations has been extensively investigated. For example, Ca²⁺ acts as an inhibitor and probably acts both within the pore ^[13] and at the binding site ^[14], although its effects may be complicated by the finding that calcineurin potentiates receptor desensitisation ^[15]. Mg²⁺ and other divalent cations, including Cd²⁺, Cu²⁺ and Zn²⁺, have also been shown to modulate 5-HT₃ receptor responses ^[16]. Like Ca²⁺, these may act at multiple sites, although the effects of Zn²⁺ appear more complex, with both inhibition and enhancement being reported ^[17].