Orphan receptor activated by neuropeptides terminating in Arg-Phe or Arg-Phe-amide. Mediates its action by association with G proteins that activate a phosphatidylinositol-calcium second messenger system. Its effect is mediated by G(q) and G(11) proteins. May regulate the function of nociceptive neurons by modulation of pain perception (By similarity). Uniquely localized in a subset of small dorsal root and trigeminal sensory neurons. Associated preferentially with IB4 class of small-diameter somatosensory afferents (also known as nociceptors). Belongs to the G-protein coupled receptor 1 family. Mas subfamily.