...There are at least 15 confirmed 5-hydroxytryptamine (5-HT) receptor subtypes, each encoded by their own distinct genes (Raymond et al., 2001). These subtypes are divided in to seven subfamilies; which are characterized by pharmacological, structural and functional features (Hoyer et al., 1994). The range of 5-HT receptors is further increased by post genomic modifications such as alternative splicing, which can give rise to 10 splice variants for the 5-HT4 receptor and mRNA editing in conjunction with 5-HT2C receptors (Bockaert et al., 2006). 5-HT1, 5-HT2, 5-HT4, 5-HT5, 5-HT6 and 5-HT7 are metabotropic G-protein coupled receptors (GPCRs), whereas 5-HT3 is uniquely ionotropic, (Engleman et al., 2008) (see Fig. 1.) Walstab, J., Rappold, G. and Niesler, B. (2010). 5-HT3 receptors: Role in disease and target of drugs. Pharmacology & Therapeutics, 128(1), pp.146-169. Fig. 1. Schematic of a typical 5-HT3 receptor, showing the cation lumen after the removal of a subunit. Arrows point to reveal the agonistic, competitive, non-competitive antagonists and positive modulators. The orthosteric ligand binding site is magnified to show the binding loops of neighbouring subunits. An arrow also shows the transmembrane (TM) domains of an individual subunit, with TM2 lining the channel pore. A pentemeric receptor consisting of four TM segments to form an intrinsic cation selective, water-filled channel (Barnes et al., 2009). A typical subunit exhibits a large extracellular N-terminus...
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