Some science behind the scenes
Enkephalin - Leu-enkephalin is an endogenous opioid peptide neurotransmitter found naturally in the brains of many animals, including humans. It is one of the two forms of enkephalin; the other is met-enkephalin. It produces pharmacological effects at both the mu and delta opioid receptors, showing selectivity for the delta receptor. It has a much higher selectivity for delta opioid receptors than mu receptors, and has little to no effect on kappa opioid receptors.
[Rothman RB, Holaday JW, Porreca F. Allosteric coupling among opioid receptors: evidence for an opioid receptor complex. In: Herz A, Akil H , Simon EJ , eds. Handbook of Experimental Pharmacology: Opioids I. Vol 104. Berlin, Germany: Springer-Verlag; 1993 :217- 237] .