Orphan receptor. Probably involved in the function of nociceptive neurons. May regulate nociceptor function and/or development, including the sensation or modulation of pain. Potently activated by enkephalins including BAM22 (bovine adrenal medulla peptide 22) and BAM (8-22)(PubMed:26582731). BAM22 is the most potent compound and evoked a large and dose-dependent release of intracellular calcium in stably transfected cells. G(alpha)q proteins are involved in the calcium-signaling pathway. Activated by the antimalarial drug, chloroquine. May mediate chloroquine-induced itch, in a histamine-independent manner. Uniquely localized in a subset of small dorsal root and trigeminal sensory neurons. Activation of this receptor requires concentrations that exceed the chloroquine concentrations observed in plasma of patients undergoing chloroquine treatment. However, chloroquine accumulates at much higher level in the skin where the receptor is located. Chloroquine-induced itch is very common among black Africans (up to 70%) but less common in other populations. Belongs to the G-protein coupled receptor 1 family. Mas subfamily.