<P> Serotonin is a neurotransmitter and is found in all bilateral animals including insects . Serotonin is also present in plants (phytoserotonin). </P> <P> Serotonin mediates the animal's perceptions of resource; In less complex animals, such as some invertebrates, resources simply mean food availability . In plants serotonin synthesis seems to be associated with stress signals . In more complex animals, such as arthropods and vertebrates, resources also can mean social dominance . In response to the perceived abundance or scarcity of resources, an animal's growth, reproduction or mood may be elevated or lowered . This may somewhat depend on how much serotonin the organism has at its disposal . </P> <P> The 5 - HT receptors, the receptors for serotonin, are located on the cell membrane of nerve cells and other cell types in animals, and mediate the effects of serotonin as the endogenous ligand and of a broad range of pharmaceutical and hallucinogenic drugs . Except for the 5 - HT receptor, a ligand - gated ion channel, all other 5 - HT receptors are G - protein - coupled receptors (also called seven - transmembrane, or heptahelical receptors) that activate an intracellular second messenger cascade . </P> <P> Serotonergic action is terminated primarily via uptake of 5 - HT from the synapse . This is accomplished through the specific monoamine transporter for 5 - HT, SERT, on the presynaptic neuron . Various agents can inhibit 5 - HT reuptake, including cocaine, dextromethorphan (an antitussive), tricyclic antidepressants and selective serotonin reuptake inhibitors (SSRIs). A 2006 study conducted by the University of Washington suggested that a newly discovered monoamine transporter, known as PMAT, may account for "a significant percentage of 5 - HT clearance". </P>

Where are serotonin receptors located in the body