5-HT3

The 5-HT₃ receptor belongs to the Cys-loop superfamily of ligand-gated ion channels (LGICs) and therefore differs structurally and functionally from all other 5-HT receptors (5-hydroxytryptamine, or serotonin) receptors which are G protein-coupled receptors. This ion channel is cation-selective and mediates neuronal depolarization and excitation within the central and peripheral nervous systems. As with other ligand gated ion channels, the 5-HT₃ receptor consists of five subunits arranged around a central ion conducting pore, which is permeable to sodium (Na), potassium (K), and calcium (Ca) 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. The rapidly activating, desensitizing, inward current is predominantly carried by sodium and potassium ions. 5-HT₃ receptors have a negligible permeability to anions. They are most closely related by homology to the nicotinic acetylcholine receptor.

The 5-HT₃ receptor differs markedly in structure and mechanism from the other 5-HT receptor subtypes, which are all G-protein-coupled. A functional channel may be composed of five identical 5-HT_3A subunits (homopentameric) or a mixture of 5-HT_3A and one of the other four 5-HT_3B, 5-HT_3C, 5-HT_3D, or 5-HT_3E subunits (heteropentameric). It appears that only the 5-HT_3A subunits form functional homopentameric channels. All other subunit subtypes must heteropentamerize with 5-HT_3A subunits to form functional channels. Additionally, there has not currently been any pharmacological difference found between the heteromeric 5-HT_3AC, 5-HT_3AD, 5-HT_3AE, and the homomeric 5-HT_3A receptor.

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