G protein-coupled receptor for 5-hydroxytryptamine (serotonin) (PubMed:10452531, PubMed:1565658, PubMed:1652050, PubMed:33762731). Also functions as a receptor for ergot alkaloid derivatives, various anxiolytic and antidepressant drugs and other psychoactive substances (PubMed:10452531, PubMed:1565658, PubMed:1652050, PubMed:33762731). Ligand binding causes a conformation change that triggers signaling via guanine nucleotide-binding proteins (G proteins) and modulates the activity of downstream effectors, such as adenylate cyclase (PubMed:10452531, PubMed:1565658, PubMed:1652050, PubMed:33762731). HTR1D is coupled to G(i)/G(o) G alpha proteins and mediates inhibitory neurotransmission by inhibiting adenylate cyclase activity (PubMed:33762731). Regulates the release of 5- hydroxytryptamine in the brain, and thereby affects neural activity (PubMed:18476671, PubMed:20945968). May also play a role in regulating the release of other neurotransmitters (PubMed:18476671, PubMed:20945968). May play a role in vasoconstriction (PubMed:18476671, PubMed:20945968). {ECO:0000269|PubMed:10452531, ECO:0000269|PubMed:1565658, ECO:0000269|PubMed:1652050, ECO:0000269|PubMed:33762731, ECO:0000303|PubMed:18476671, ECO:0000303|PubMed:20945968}. This is the function of HTR1D (5-hydroxytryptamine receptor 1D, Ensembl gene identifier ENSG00000179546).