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. Regulates the release of 5-hydroxytryptamine in the brain, and thereby affects neural activity. May also play a role in regulating the release of other neurotransmitters. May play a role in vasoconstriction. Homodimer. Heterodimer with HTR1B (By similarity). Detected in the motor column in spinal cord, and in several cranial motor nuclei, including nucleus ambiguous, oculomotoris, trochelaris and abducens. Detected in gamma motor neurons in the lumbar spinal cord. Detected in proprioceptive sensory neurons in dorsal root ganglia. No visible phenotype. Depending on the task, mutant mice show improved motor coordination, especially in avoiding hind limb slips when crossing a narrow beam and in climbing onto a horizontal hanging wire. Belongs to the G-protein coupled receptor 1 family.