Leu-Enkephalin is an endogenous neuropeptide involved in nociception and an agonist of δ- and ?-opioid receptors (K_is = 4.0 and 3.4 nM, respectively).^1,2It is selective for δ- and ?- over κ-opioid receptors (K_i= >1,000 nM).²Leu-Enkephalin is a cleavage product of proenkephalin, which is found primarily in the adrenal medulla and the CNS.³It is found in the brain, the brainstem, and the spinal cord as well as in the peripheral nervous system. Leu-Enkephalin (1 mg/kg) decreases the visceromotor response to colon distension in a mouse model of visceral pain induced by colorectal distension.⁴It also increases the tail-flick threshold and latency to tail flick in the tail-pressure test and tail-flick test, respectively, in mice.⁵

1.Hughes, J., Smith, T.W., Kosterlitz, H.W., et al.Identification of two related pentapeptides from the brain with potent opiate agonist activityNature258(5536)577-580(1975) 2.Raynor, K., Kong, H., Chen, Y., et al.Pharmacological characterization of the cloned κ-, δ-, and μ-opioid receptorsMol. Pharm.45(2)330-334(1994) 3.Roques, B.P., Noble, F., and Fourni?-Zaluski, M.-C.Endrogenous opioid peptides and analgesiaOpioids in pain control: Basic and clinical aspects22-25(1999) 4.Fabisiak, A., Sobociska, M., Kamysz, E., et al.Antinociceptive potency of enkephalins and enkephalinase inhibitors in the mouse model of colorectal distension-proof-of-conceptChem. Biol. Drug Des.92(1)1387-1392(2018) 5.Honda, M., Okutsu, H., Matsuura, T., et al.Spinorphin, an endogenous inhibitor of enkephalin-degrading enzymes, potentiates leu-enkephalin-induced anti-allodynic and antinociceptive effects in miceJpn. J. Pharmacol.87(4)261-267(2001)