Receptor for NPAF (A-18-F-amide) and NPFF (F-8-F-amide) neuropeptides, also known as morphine-modulating peptides. Can also be activated by a variety of naturally occurring or synthetic FMRF-amide like ligands. This receptor mediates its action by association with G proteins that activate a phosphatidylinositol-calcium second messenger system. Experimental confirmation may be lacking for some isoforms. Isoform 1 is abundant in placenta. Relatively highly expressed in thymus, testis, and small intestine. Expressed at low levels in several tissues including spleen, prostate, brain, heart, ovary, colon, kidney, lung, liver and pancreas and not expressed in skeletal muscle and leukocytes. Isoform 2 expression is highest in placenta (but at relatively low level compared to isoform 1). Very low level of expression in numerous tissues including adipose tissue and many brain regions. Isoform 3 is expressed in brain and heart and, at lower levels, in kidney, liver, lung and pancreas. Belongs to the G-protein coupled receptor 1 family.