SR-5B, also designated 5-hydroxytryptamine (serotonin) receptor 5B (5-HT5B) and G protein-coupled receptor 134 (GPR134), is a receptor for the monoamine ligand serotonin (5-hydroxytryptamine, 5-HT). Serotonin is a neurotransmitter derived from serotonergic neurons in the central nervous system and entero-chromaffin cells in the gastrointestinal tract. Serotonin actions are mediated by receptors that influence the biochemistry of depression, anxiety, sexuality and appetite. Rat SR-5B is present in serotonergic neurons in dorsal raphe (DR) and central superior nucleus (CS, median raphe nucleus). DR cell bodies showing SR-5B mRNA expression are abundant in the medial portions of the nucleus. CS coexpression of SR-5B receptor mRNA with serotonin transporter mRNA is high in the intermediate portions of the nucleus. Serotonin receptors include SR-1–7 (5-HT1–7). Subtypes within the SR-1 group include SR-1A, -1B, -1D, -1E and -1F. Subtypes within the SR-2 group include SR-2A, -2B and -2C. Subtypes within the SR-5 group include SR-5A and -5B. SR receptors can couple to G proteins that act on either adenylate cyclase or phospholipase C (PLC). The SR-3 class of receptors are ion channels.