G-protein coupled receptor for 5-hydroxytryptamine (serotonin). Also functions as a receptor for various ergot alkaloid derivatives and psychoactive substances. Ligand binding causes a conformation change that triggers signaling via guanine nucleotide-binding proteins (G proteins) and modulates the activity of down-stream effectors. Beta-arrestin family members inhibit signaling via G proteins and mediate activation of alternative signaling pathways. Signaling activates a phosphatidylinositol-calcium second messenger system that modulates the activity of phosphatidylinositol 3-kinase and down-stream signaling cascades and promotes the release of Ca(2+) ions from intracellular stores. Plays a role in the regulation of dopamine and 5-hydroxytryptamine release, 5-hydroxytryptamine uptake and in the regulation of extracellular dopamine and 5-hydroxytryptamine levels, and thereby affects neural activity (By similarity). Detected in brain, heart and gut. Ligands are bound in a hydrophobic pocket formed by the transmembrane helices. Belongs to the G-protein coupled receptor 1 family.