G-protein coupled receptor for 5-hydroxytryptamine (serotonin). Also functions as a receptor for various alkaloids. 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 activates adenylate cyclase activity. Expressed predominantly in adult heads. Belongs to the G-protein coupled receptor 1 family. 5-hydroxytryptamine receptor subfamily.