Pharmacological studies as well as molecular cloning of serotonin receptors have revealed a multiplicity of receptor subtypes not only in mammals but also in molluscs and arthropods. Most of these receptors belong to the G protein-coupled receptor family and modulate levels of second messengers such as cAMP, IP3 and calcium. Rather than being specialized in a particular physiological function, a given receptor may be expressed in multiple neurons throughout the brain but always in the same compartment of these neurons. The 5HT1B receptor for example, is generally localized presynaptically on neuronal terminals where it inhibits neurotransmitter release. A widespread distribution of 5-HT receptors might explain how serotonin can modulate the multiple neuronal circuits which underlie complex behaviors.