Prolactin Releasing Peptide (PrRP) has been identified as a specific, high affinity endogenous ligand for GPR10. Prolactin Releasing Peptide (PrRP) preproprotein can be cleaved at two different positions to give rise to two forms of 31 or 20 amino acids; Prolactin Releasing Peptide (PrRP)-31 and Prolactin Releasing Peptide (PrRP)-20 respectively. Rat Prolactin Releasing Peptide (PrRP) has also been identified and occurs as 31 or 20 amino acid forms; these peptides are highly conserved between species. Human PrRP-20, human PrRP-31, rat PrRP-20 and rat PrRP-31 display high affinity for GPR10 receptors, with Ki values of 0.26±0.07, 1.03±0.41, 0.22±0.06 and 0.33±0.11 nM, respectively.