G-protein coupled receptor for 5-hydroxytryptamine (serotonin). Also functions as a receptor for various drugs and psychoactive substances, including mescaline, psilocybin, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and lysergic acid diethylamide (LSD). 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 phospholipase C and a phosphatidylinositol-calcium second messenger system that modulates the activity of phosphatidylinositol 3-kinase and promotes the release of Ca(2+) ions from intracellular stores. Affects neural activity, perception, cognition and mood. Plays a role in the regulation of behavior, including responses to anxiogenic situations and psychoactive substances. Plays a role in intestinal smooth muscle contraction, and may play a role in arterial vasoconstriction. Interacts (via C-terminus) with MPDZ and PATJ. May interact (via C-terminus) with MPP3, PRDX6, DLG4, DLG1, CASK, APBA1 and MAGI2. Interacts with GRM2 and DRD2; this may affect signaling. Detected in adult intestine, especially in mucosal epithelium, longitudinal and circular layers of muscularis externa and myenteric plexuses. Highly expressed in Paneth cells, and detected at lower levels in enterocytes (at protein level). The PDZ domain-binding motif is involved in the interaction with PATJ, CASK, APBA1, DLG1 and DLG4. Belongs to the G-protein coupled receptor 1 family.