Signaling protein that functions as an antagonist of melanocyte-stimulating-hormone receptors MC3R and MC4R by precluding agonist-induced signaling, thereby inhibiting cAMP production within the hypothalamus and adrenal gland (PubMed:10371151, PubMed:11145747, PubMed:9311920, PubMed:9892020). Involved in the control of feeding behavior via the central melanocortin system (PubMed:15927146, PubMed:9311920). Has very low activity towards MC5R (PubMed:9311920, PubMed:9892020). Also promotes endocytosis of MC3R and MC4R in an arrestin-dependent mechanism (PubMed:17041250). {ECO:0000269|PubMed:10371151, ECO:0000269|PubMed:11145747, ECO:0000269|PubMed:15927146, ECO:0000269|PubMed:17041250, ECO:0000269|PubMed:9311920, ECO:0000269|PubMed:9892020}. This is the function of ENSG00000159723 (AGRP, agouti related neuropeptide).