5-HT receptors, 5-hydroxytryptamine receptors, or serotonin receptors, are a group of G protein-coupled receptor and ligand-gated ion channels found in the central and peripheral nervous systems. They mediate both excitatory and inhibitory neurotransmission. The serotonin receptors are activated by the neurotransmitter serotonin, which acts as their natural ligand. Serotonin receptors are found in almost all animals and are even known to regulate longevity and behavioral aging in the primitive nematode, Caenorhabditis elegans.