Dynorphin A (1-13) amide is an endogenous opioid peptide that antagonizes morphine-induced analgesia[1].
References
[1] Woo S,et al. Dynorphin(1-10)amide: a potent and selective analog of dynorphin(1-13). Life Sci. 1982 Oct 18-25;31(16-17):1817-20. DOI:10.1016/0024-3205(82)90218-1