[Leu5]-Enkephalin, amide is a δ opioid receptor agonist. δ opioid receptor[1]

[Leu5]-Enkephalin causes concentration-dependent, reversible inhibition of pelvic nerve-evoked contractions, with an IC50 value of 2.1 nM[2].

Levels of [Leu5]-Enkephalin are significantly increased in the nucleus raphe magnus (NRM) of rats 2 weeks after the injection of complete Freund's adjuvant (CFA) (1.02±0.2 pmol/mg protein) as compared with saline-treated rats (0.49±0.04 pmol/mg protein; p<0.01). Tissue levels of [Leu5]-Enkephalin are uniformly increased in the caudal ventrolateral periaqueductal gray (PAG) 4 hr (1.15±0.25 pmol/mg protein), 4 d (1.16±0.18 pmol/mg protein), and 2 weeks (1.18±0.17 pmol/mg protein) after the injection of CFA as compared with saline-treated rats (0.55±0.03 pmol/mg protein; p<0.05, all times). A smaller increase in the levels of [Leu5]-Enkephalin occurred in the rostral aspect of the ventrolateral PAG at all time points. Finally, levels of [Leu5]-Enkephalin are also increased in the contralateral microcellular tegmental nucleus 4 d after the injection of CFA (0.53±0.04 pmol/mg protein) compared with levels in saline-treated rats (0.38±0.02 pmol/mg protein; p<0.05)[1].

[1]. Hurley RW, et al. Contribution of endogenous enkephalins to the enhanced analgesic effects of supraspinal mu opioid receptor agonists after inflammatory injury. J Neurosci. 2001 Apr 1;21(7):2536-45. [2]. Kennedy C, et al. [Met5]enkephalin acts via delta-opioid receptors to inhibit pelvic nerve-evoked contractions of cat distal colon. Br J Pharmacol. 1987 Oct;92(2):291-8.