[Leu5]-Enkephalin, amide

[Leu5]-Enkephalin, amide is a δ opioid receptor agonist. IC50 & Target: δ opioid receptor^[1] In Vitro: [Leu5]-Enkephalin causes concentration-dependent, reversible inhibition of pelvic nerve-evoked contractions, with an IC₅₀ value of 2.1 nM^[2]. In Vivo: 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].