[Met5]-Enkephalin, amide TFA is an agonist for δ opioid receptors as well as putative ζ (zeta) opioid receptors. δ and ζ opioid receptor[1]
[Met5]-Enkephalin at 0.1 nM, 10 nM, and 1 μM significantly reduces the total number of glial cells in culture[1]. [Met5]-Enkephalin, amide acts via δ-opioid receptor to inhibit pelvic nerve-evoked contractions of cat distal colon. [Met5]-enkephalin causes concentration-dependent, reversible inhibition of pelvic nerve-evoked contractions, with an IC50 value of 2.2 nM. [Met5]enkephalin at a concentration (3 nM) which produces a large inhibition of neurogenic contractions, has no effect on contractions to exogenous acetylcholine[2].
[1]. Stiene-Martin A, et al. Glial growth is regulated by agonists selective for multiple opioid receptor types in vitro. J Neurosci Res. 1991 Aug;29(4):538-48. [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.