5-Hydroxytryptamine (5-HT) receptors consist of at least three distinct types of molecular structures (Table 1): guanine nucleotide binding protein (G protein)-coupled receptors, ligand-gated ion channels, and transporters. Prior to the introduction of molecular biological techniques, the classification of 5-HT receptors was based predominantly on the pharmacological properties of the receptors. For example, “5-HT1” receptors were defined as membrane binding sites that displayed nanomolar affinity for [3H]5-HT (Peroutka and Snyder, 1979). Subsequently, Bradley et al. (1986) defined “5-HT1-like” receptors by their susceptibility to antagonism by methiothepin and/or methysergide, resistance to antagonism by 5-HT2 antagonists, and potent agonism by 5-carboxamidotryptamine (5-CT). Thus, these classification systems are dependent upon the availability of selective pharmacological agents.