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. Detected in the brain with the greatest abundance in the hippocampus, followed by the olfactory bulb. Lower levels are detected in the cortex, thalamus, pons, hypothalamus, midbrain, striatum, and cerebellum. Belongs to the G-protein coupled receptor 1 family.