G-protein coupled receptor for 5-hydroxytryptamine (serotonin). Also functions as a receptor for various alkaloids 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, such as adenylate cyclase. Signaling inhibits adenylate cyclase activity. Arrestin family members inhibit signaling via G proteins and mediate activation of alternative signaling pathways. Regulates the release of 5-hydroxytryptamine, dopamine and acetylcholine in the brain, and thereby affects neural activity, nociceptive processing, pain perception, mood and behavior. Besides, plays a role in vasoconstriction of cerebral arteries. Homodimer. Heterodimer with HTR1D (By similarity). Ligands are bound in a hydrophobic pocket formed by the transmembrane helices. Phosphorylated. Palmitoylated. A residue in the 7th transmembrane region ('Thr-355' in human, Asn-351 in mouse and rat) is important for species-specific sensitivity to various agonists. Belongs to the G-protein coupled receptor 1 family.