The 5-HT3 receptor is formed by five subunits arranged around a pore forming unit. Receptors could be either monomeric, such as 5-HT3A or heteromeric entities like 5-HT3A/B. Indeed, the type of channel formed displays different pharmacological and electrophysiological characteristics. To date, five 5-HT3 subunits have been identified 5-HT3A-E, which show variability in the N-terminus and in the transmembrane region. 5-HT3A and 5-HT3B are the best characterized among the different types.