G protein-coupled receptor for 5-hydroxytryptamine (serotonin), a biogenic hormone that functions as a neurotransmitter, a hormone and a mitogen (PubMed:35610220, PubMed:35835867, PubMed:9865521). Also functions as a receptor for ergot alkaloid derivatives and other psychoactive substances (PubMed:35835867). Ligand binding causes a conformation change that triggers signaling via guanine nucleotide-binding proteins (G proteins) and modulates the activity of downstream effectors (PubMed:35610220, PubMed:35835867, PubMed:9865521). HTR5A is coupled to G(i)/G(o) G alpha proteins and mediates inhibitory neurotransmission: signaling inhibits adenylate cyclase activity and activates a phosphatidylinositol-calcium second messenger system that regulates the release of Ca(2+) ions from intracellular stores (PubMed:35610220, PubMed:35835867, PubMed:9865521). {ECO:0000269|PubMed:35610220, ECO:0000269|PubMed:35835867, ECO:0000269|PubMed:9865521}. This is the function of Ensembl gene identifier ENSG00000157219 (HTR5A, 5-hydroxytryptamine receptor 5A).